(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
Animals lack the specific enzymes needed to break down coal, so no energy can be released from it for use by the body.
Both statements are correct and directly linked. The textbook states that heterotrophs use enzymes (bio-catalysts) to break down complex food substances. Coal is not a biological food material, so no enzyme exists to digest it, making energy extraction impossible. R correctly explains A — hence option (A) is the answer. Watch for traps: option (B) applies when both are true but unrelated; here the link is direct.