Autotrophic organisms refer to organisms that can synthesize organic matter by themselves, including photoautotrophic organisms and chemoautotrophic organisms. Photoautotrophs use light energy to synthesize organic matter through photosynthesis, while chemoautotrophs can use chemical energy to synthesize organic matter. For example, nitrifying bacteria can use chemical energy to synthesize organic matter. So choose C.

Test center: This question examines the concept of autotrophs.

Comments: This question is intended to examine the understanding and judgment ability of candidates, and it is a moderately difficult question.