B, in the food chain of the ecosystem, the level of trophic level is not necessarily related to the size of the body, such as the size of wolves with high trophic level and buffaloes with low trophic level, B is wrong;

C, the trophic level of each animal in different food chains in the food web may be different, so each animal is not necessarily in a trophic level in the ecosystem, and C is wrong;

D, bacteria and fungi living in saprophytes are decomposers, some bacteria are producers, such as cyanobacteria, and some may be consumers, such as Mycobacterium tuberculosis in human body, D error.

So choose: a.