B, nitrifying bacteria can't use nitrous acid and nitric acid as their own components, and B is wrong;

C, nitrifying bacteria are classified as autotrophs, because it can use the energy released when substances are oxidized to synthesize organic matter from carbon dioxide in the air for its own use, and C is correct;

D, nitrifying bacteria can use the chemical energy released in the process of ammonia oxidation to synthesize organic matter, instead of directly synthesizing ATP, D is wrong.

So choose: C.