Look at the launch year first. All four satellites in the options went up close to 2015, so the deciding clue is the specific feature the question describes, precise timing down to the 1000th of a second.
GSAT-6, launched on 27 August 2015 aboard GSLV-D6, was built for strategic S-band mobile communication and needed very fine signal timing and synchronization to work correctly, which lines up with the description in the question.
GSAT-16 was a plain transponder-capacity satellite, IRNSS-1D was a navigation satellite for positioning, and Astrosat was a scientific observatory for astronomy, so options A, B and D do not match the timing feature asked about.