Option D: Assertion (A) is false, but Reason (R) is true.
Agricultural production activities belong to the primary sector, not the tertiary sector. However, the Reason is correct — the tertiary sector is indeed called the service sector.
The key distinction: Primary sector = natural/agricultural production; Tertiary sector = services (transport, banking, education, etc.). The Assertion wrongly places agriculture in the tertiary sector, making it false. The Reason independently states a correct fact about the tertiary sector, so Option D is correct.