(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
Carbon compounds are major fuel sources because most of them release a large amount of heat and light on burning (combustion), which directly explains why they are used as fuels.
The key here is the word "correct explanation" — R directly explains why A is true. The textbook states both facts together under Combustion (4.3.1): carbon compounds are fuels because they release heat and light on burning. So R is not just true but is the precise reason for A, making option (A) correct. Watch out for option (B) traps where both statements are true but unrelated — here they are clearly linked.