The fundamental difference between heterotrophic and autotrophic metabolic assimilation lies in (a) whether photosynthesis can be carried out, (b) whether light energy can be used, and (c) whether it is beneficial. First, autotrophs can also carry out chemical synthesis, such as nitrifying bacteria, a mistake;

B, autotrophs can also use chemical energy, such as nitrifying bacteria, B error;

C, autotrophs can also use light energy, C is wrong;

D the difference between autotrophs and heterotrophs is that autotrophs can use inorganic carbon to synthesize organic carbon, while heterotrophs can only use organic carbon, and inorganic carbon cannot synthesize organic carbon. D is correct.

Therefore, choose: d.