(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A), because pesticides accumulate at each trophic level (biological magnification), reaching maximum concentration at the highest level.
Source: Our Environment, 13.1.1
The passage directly states that harmful chemicals "get accumulated progressively at each trophic level" (this is biological magnification). Humans/top consumers accumulate the maximum concentration. Since R correctly explains why A is true, option (A) is the answer. Note: the passage says humans at the top level get maximum concentration — tertiary consumers are at/near the top, so A holds true.