(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
India adopted the multi-party system because its vast social, regional, and geographical diversity requires multiple parties to adequately represent all sections of society.
The Reason directly explains why India has a multi-party system — the need to accommodate diverse social and geographical differences across a large federal country. Since both statements are true and R logically explains A, option (A) is correct. Remember: in assertion-reason questions, check (1) are both true? (2) does R actually explain A?