Which one of the following is a reason why astronomical distances are measured in light-years?

(a) Distances among stellar bodies do not change.
(b) Gravity of stellar bodies does not change.
(c) Light always travels in straight line.
(d) Speed of light is always same. ✓ UPSC's answer

Why the answer is (d)

• A light-year is the distance light travels in one year (about 9.46 trillion km); it is a reliable unit because the speed of light in vacuum is a universal constant, identical everywhere and at all times — option (d).

• Because distances between stars are so vast, kilometres become unwieldy; light-time gives manageable numbers (Proxima Centauri ≈ 4.2 light-years).

• Stellar distances do change (a) and gravity varies (b); light bends near massive bodies (c), so those are not the reasons.

• Hence option (d).

Why the other options are wrong

(a) Distances among stellar bodies do not change.: Stellar distances do change over time.
(b) Gravity of stellar bodies does not change.: Gravity of stellar bodies varies and is irrelevant to the unit.
(c) Light always travels in straight line.: Light bends near massive objects; this is not why light-years are used.

Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2021, held on 10 October 2021. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.