In grazing food chain (GFC), producers constitute the first trophic level whereas detritus food chain  (DFC) begins with detritus or dead organic matter.

In the given food web, organisms I (phytoplanktons) act as producers, organisms II, V and VII  act as primary consumers as they feed on producers, organisms III and VI act as secondary consumers as they upon primary consumers, and organism IV acts as a tertiary consumer.

Less that 50% of the solar energy incident over earth is PAR (photosynthetically active radiation).

About 1 −5% of incident solar energy or 2 −10% of PAR is captured by the photosynthetic organisms for the synthesis of organic matter(Gross primary productivity). Roughly 20% of it is consumed in respiration so that net capture of energy(net primary productivity) is 0.8 −4% of incident radiation or 1.6 −8% of PAR.

Decomposers and  detritivores constitute the first trophic level of a detritus food chain.

Only 10 % of energy is transferred from one trophic level to the next trophic level of a food chain (ten percent law, given by Linderman). In present case, energy available at producer level is 10 J. Hence energy at primary consumer level would be 1 H (10% of 10 J). Similarly, at secondary consumer level, energy present will be 0.1 J (100 % of 1 J).

Trophic level is a functional level. A single species may occupy more than one trophic level. Insectivorous plants are producers, occupying first trophic level. They also eat insects and thus, occupy third trophic level also.

If  population of hawks would increase, they would feed on more grasshoppers which will result in decrease of grasshoppers population, so option C is wrong

In a grassland ecosystem, the pyramid of numbers is upright. If primary producers are approximately 6 million plants, then number of top carnivores must be less than 6 milliion. Hecne, according to the option given, there would be 3 million top carnivores.

A single species may occupy more than one trophic level. Sparrow being omnivorous can be a primary consumer if it feeds on seeds, fruits and peas or a secondary consumer if it feeds on insects and worms.

If carnivores, which feed on deers are removed, then the deer population will flourish. Removal of 80% of tigers from an area will cause increase in population of herbivores, which would further feed more on vegetation, decreasing its growth. The length of food chain is limited to 3-4 trophic levels as only 10% of energy is transferred to next trophic level. Hence after 3-4 trophic levels there is very little or no energy left to sustain more trophic levels. 

A lion feeds on a deer.
Deer are the primary consumer.
Fox acts a secondary consumer and feeds on a primary consumer such as Rabbit.
Frog feeds on a grasshopper and acts as a primary consumer.
Rat acts as prey for both Snake and a hawk.

In an aquatic ecosystem, the important herbivores are zooplankton, larvae, tadpoles, etc. Cows in grasslands also act as herbivores as these feed on producers. Thus, both cows and zooplankton occupy second trophic level in terrestrial and aquatic ecosystem respectively.

First-order consumers are animals that eat plants called herbivores. They are the first step in the food chain.