(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
India's social and geographical diversities led to the rise of many parties representing different groups, making it a multiparty system.
The textbook explains that India has a multiparty system because no single party can represent the vast social, regional, and cultural diversity of the country. The Reason directly explains why the Assertion is true, so option (a) is correct. Remember: when R logically explains A, choose (a), not (b).