Which of the following are not used as respiratory substrates?                       

The organic nutrients oxidized at the time of cellular respiration are otherwise called as  Respiratory substrates.During cellular respiration, oxidation of organic nutrients leads to the production of energy currency of the cell, i.e, ATP.

Usually carbohydrates are oxidized to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions.

Nucleic acids are not used as respiratory substrates.