(b) Both A and R are true, but R is not the correct explanation of A. A is explained by biological magnification — chemicals accumulate at each trophic level, so the top level has maximum concentration.
The Assertion is true — biological magnification causes maximum accumulation at the highest trophic level. The Reason is also true — pesticides/chemicals are indeed sprayed on crops. However, R only states the source of chemicals entering the food chain, not why they accumulate most at the top trophic level. The correct explanation of A is biological magnification (progressive accumulation at each trophic level), not merely that chemicals are sprayed. Hence option (b).