Reduction is referred to as the gain of electrons during a reaction by a molecule, atom, or ion. During reduction, the oxidation number decreases. When oxidation and reduction take place simultaneously, it’s called a redox reaction.

A molecule or a compound can act as both oxidising agent as well as reducing agent if the oxidation state of central metal atom lies between its stable oxidation states. As in case of HNO2, the oxidation state of Nitrogen is +3 which is between 0 and +5 which are its stable oxidation states.

The above reaction is a redox reaction where both oxidation and reduction are taken place at the same time. The reducing agent is Carbon monoxide (CO) as it gains an oxygen atom and reduces other compounds to Fe. Fe in Fe₂O₃ is reduced by CO.

Oxidation or Reduction reaction also known as redox reactions involves change in the oxidation state of the involved species but in case of option c, there is no change in the oxidation states of Hydrogen, Phosphorous and Oxygen. This reaction is just combination reaction and not a redox reaction. Hence, option C is correct

 Autoxidation is any oxidation that occurs in presence of oxygen. The term is usually used to describe the degradation of organic compounds in air (as a source of oxygen). Autoxidation produces hydroperoxides and cyclic organic peroxides. These species can react further to form many products. The process is relevant to many phenomena including aging, paint, and spoilage of foods, degradation of petrochemicals, and the industrial production of chemicals. Autoxidation is important because it is a useful reaction for converting compounds to oxygenated derivatives, and because it occurs in situations where it is not desired (as in the destructive cracking of the rubber in automobile tires or in rancidification).

Water automatically gets oxidised to hydrogen peroxide.