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The digestion of proteins and nucleic acids produces toxic wastes that contain nitrogen. These wastes have to be eliminated from our bodies as they are poisonous. The three types of nitrogenous wastes excreted are ammonia, urea, and uric acid depending on the type of organism.

Nitrogen is an important element that comprises 78% of the Earth's atmosphere. It plays an important role in the growth and development of plants and is a component of proteins and nucleic acids. The nitrogen excreted from the body of organisms is in the form of nitrogen compounds such as urea, uric acid, and ammonia. Hence, option a is incorrect.

Ammonia is the most toxic nitrogenous waste and is produced in aquatic animals such as fish. Ammonia requires a lot of water to be expelled from the body, and fish can easily eliminate it as they are surrounded by water. Hence, option b is incorrect.

Uric acid is the nitrogenous waste produced by reptiles, birds, and insects. It is the least toxic of the nitrogenous wastes and requires very little water to be excreted from the body. Hence, option d is incorrect.

Urea is the excretory product found in mammals such as human beings. It is moderately toxic and requires a moderate amount of water to be expelled from the body. The digestion of proteins produces ammonia which is converted into urea before being excreted in the urine. Hence, option c is correct.