Option 1 suggests that both the assertion and the reason are accurate, and the reason correctly explains the assertion. However, it`s incorrect because, while Harshavardhana did convene the Prayag Assembly, it wasn`t for the sole purpose of promoting the Mahayana form of Buddhism.

Option 2, which is indeed the correct answer, also affirms that both the assertion and reason are individually accurate but clarifies that the reason doesn`t correctly elucidate the assertion. Harshavardhana did hold the Prayag Assembly, yet it wasn`t only to promote the Mahayana form of Buddhism.

Option 3 posits that the assertion is true and the reason is false. This is not accurate since, though Harshavardhana did gather the Prayag Assembly, wishing to propagate just the Mahayana Buddhism isn`t incorrect.

Option 4 implies the assertion is false, but the reason is true. This is incorrect because both the assertion about Harshavardhana convening the Prayag Assembly is true, and the reason associating it to Mahayana Buddhism is also true even if it doesn`t explain the assertion.