Thiamin or vitamin B₁ is an important water soluble vitamin. Vitamin B₁ plays an essential role in cellular metabolism as coenzyme thiamine pyrophosphate. Beriberi is a deficiency disease which develops due to deficiency of vitamin B₁. It is common in individuals who consume polished grains and refined flour because whole grains are a good source of vitamin B₁. The important symptoms of beriberi are loss of appetite. fatigue, muscle atrophy, paralysis, mental confusion, cardiac oedema etc.

Vitamin D, also known as calciferol, comprises a group of fat-soluble seco-sterols. The two major forms are vitamin D2 and vitamin D3.

Vitamin E is a fat-soluble vitamin. Vitamin E is also known as antisterility vitamin. It is required for motility of sperms, maturation of RBCs and is an important antioxidant vitamin. The vitamin E deficiency causes problems such as poor transmission of nerve impulses, muscle weakness, sterility and degeneration of the retina that can cause blindness.

Vitamin B₁ is called as thiamine. The deficiency of this vitamin results in beriberi. It affects the nervous and the circulatory system.
Vitamin D deficiency results in rickets. Bending of the long bones is a common symptom. Due to deficiency, the bones become brittle and fractures or breaking of the bones is also observed.
Pellagra is a disease caused by a lack of the vitamin niacin (vitamin B₃). Symptoms include inflamed skin, diarrhea, dementia, and sores in the mouth. Primary pellagra is due to a diet that does not contain enough niacin and tryptophan.
Vitamin K deficiencies in newborns are associated with vitamin K deficiency bleeding or VKDB (also known as hemorrhagic disease of the newborn). This can cause excessive bleeding and bruising and, in severe cases, can lead to fatal bleeding into the brain, but not sterility.

Bow- shaped legs are caused by a vitamin D deficiency also called rickets. Babies are born bowlegged because of their position in the womb. By age 3, most kids no longer appear bowlegged.

Vitamin K refers to a group of fat-soluble vitamins that play a role in blood clotting, bone metabolism, and regulating blood calcium levels. The body needs vitamin K to produce prothrombin, a protein and clotting factor that is important in blood clotting and bone metabolism.

Pellagra is characterised by swollen lips, pigmented skin of hands, legs and irritability. This disease is caused by the deficiency of vitamin-B, of niacin. This is pellagra protective vitamin and can be synthesised in the body from amino acid tryptophan.

When your skin is exposed to sunlight, it makes vitamin D from cholesterol. The sun's ultraviolet B (UVB) rays hit cholesterol in the skin cells, providing the energy for vitamin D synthesis to occur. Vitamin D has many roles in the body and is essential for optimal health.

Dental fluorosis, also termed as mottled enamel, is an extremely common disorder. It is caused by overexposure to fluoride during the first eight years of life. This is the time when most permanent teeth are being formed. Clinical dental fluorosis is characterized by staining and pitting of the teeth. In more severe cases all the enamel may be damaged.. The early symptoms of skeletal fluorosis include stiffness and pain in the joints.  Hence, the dental disease characterised by mottling of teeth is due to the presence of an ingredient in drinking water is fluorine.

Pellagra is a vitamin deficiency disease most frequently caused by a chronic lack of niacin (vitamin B3) in the diet. Pellagra is characterized by diarrhoea, dermatitis, dementia etc. Thus, the correct answer is option C.

alpha- Tocopherol accounts for 90% of the vitamin E in human tissues and acts as an antioxidant. Vitamin E protects cell membranes, proteins and DNA from oxidation and thereby contributes to cellular health.

Beta-carotene (β-carotene) is a precursor to vitamin A, an essential vitamin at any age, including for cellular health and vision. It is also a powerful antioxidant that may reduce the risk of cancer.

Pantothenic acid and biotin are the types of vitamin B complex. They are water-soluble. Pantothenic acid and biotin are essential for growth. They help the body to break down and use food (metabolism). Pantothenic acid also plays a role in the production of hormones and cholesterol.

Cellulose is a polysaccharide. It is formed by the linkage of beta 1,4 linkage of the glucose unit. It is an important component of the cell wall of the plant. It is used for the production of paper. It does not have free aldehyde group so they are non reducing sugar. It is a fibrous polysaccharide as it exists in form of long chain-like structure and is mainly found in plants and most abundant organic compound on earth.
So, the correct answer is option B.

Molybdenum is required for nitrogen fixation in plants. Nitrate metabolism depends upon enzyme nitrate reductase. It is an activator of nitrate reductase. therefore, the element is important for nitrogen fixation.

The increase in enzyme activities correlates with highest mercury accumulation in the kidneys and liver. Mercury generates reactive oxygen species (ROS). Therefore, the enzyme activities increased to scavenge ROS levels produced as a result of mercury accumulation. The kidneys accumulate the highest levels of mercury compared to brain and liver.

Vitamin B₁ (thiamine) is a co-factor for pyruvate dehydrogenase, an essential enzyme for aerobic metabolism. In the absence of thiamine, the conversion of pyruvate to acetyl-CoA is inhibited and pyruvate cannot enter the Kreb's cycle.

Citrus fruits have Citric acid and along with it, they also have reasonable amount of Ascorbic acid and Vitamin C.
It is destroyed by heat and is not well stored in the body. Ascorbic acid is a good reducing agent and facilitates many metabolic reaction.
Ants (also known as formicae farris in latin) have formic acid/Methanoic acid (HCOOH) in their sting.
Spinach leaves have oxalic acid in them and it can interfere with calcium absorption.Lemons have citric acid in them.
Ascorbic acid is present in citrus fruits. Hence, the correct answer is option (A).

Vitamins are categorized as fat-soluble and water-soluble. Vitamin B and C are considered as water-soluble. While, vitamins A, D, E and K are fat-soluble vitamins, which means they are soluble in fats (lipids). 

The deficiency of vitamin A results in night-blindness, deficiency of vitamin K causes Bleeding disorders.

So, the correct option is D.