If a substance accumulates in an animal rather than being broken down, then at each trophic level the concentration of that pesticide increases in a process called

The process should be called biomagnification.

Biomagnification occurs because if one animal has some toxic substances in its body like pesticide, a larger animal that that has a higher trophic level than the smaller one will consume the smaller animal, and that the toxic substance is consumed too. However, the larger animal does not only consume one smaller animal, but a lot more. Therefore, if all the smaller animals have the toxic substance, the level of the toxic substance magnifies in the larger animal.

This cycle continues by a even larger animal eats more of the large animal and so the toxic level is even further magnified for the even larger animal. This goes on and on until the toxic level has reached a level that can kill the animal.

Biomagnification is often mixed up with bioaccumulation. Bioaccumulation happens in one organism, instead of the whole food chain. Bioaccumulation is where the toxic substance accumulates in only the organism. Since the substance cannot be excreted away. But bioaccumulation does not involve the higher trophic levels.

Therefore, your answer should be biomagnification.