(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
The textbook directly states: "There are generally a greater number of individuals at the lower trophic levels of an ecosystem, the greatest number is of the producers."
The Reason directly explains the Assertion — producers (first/lowest trophic level) are the most numerous, which is why lower trophic levels have greater numbers. Both statements appear together as a single line in the textbook (Chapter 13, Food Chains and Webs), making (A) the correct choice.