Assertion (A): PSLV is ISRO's reliable mainstay launch vehicle.
Reason (R): PSLV's flexibility allows multi-payload launches to multiple orbits with excellent reliability.

The correct answer is Both true, R explains A.

1. Analyzing Assertion (A):
Polar Satellite Launch Vehicle (PSLV) is known as the trusted "workhorse" or mainstay launch vehicle of the Indian Space Research Organisation (ISRO). It has consistently delivered numerous Indian and international satellites into orbit with an exceptionally high success rate. Thus, Assertion (A) is true.

2. Analyzing Reason (R):
PSLV is designed with high adaptability, capable of placing multiple payloads into diverse orbits (such as Sun-Synchronous Polar Orbits, Low Earth Orbits, and Geostationary Transfer Orbits) within a single mission. This extraordinary operational flexibility and track record of precision directly explain why it serves as ISRO's reliable mainstay. Thus, Reason (R) is true and correctly explains Assertion (A).