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

Answer: D. The SPEED OF LIGHT IS ALWAYS THE SAME (in vacuum) — it is the universal physical constant that makes the light-year a useful unit. A LIGHT-YEAR is the DISTANCE LIGHT TRAVELS IN ONE YEAR THROUGH VACUUM. The light-year ≈ 9.461 × 10^15 metres (~9.46 trillion km). Astronomers use it because interstellar and intergalactic distances are vast (Proxima Centauri ~4.24 light-years; Andromeda galaxy ~2.5 million light-years). WHY IT WORKS: The SPEED OF LIGHT IN VACUUM (c) is a fundamental PHYSICAL CONSTANT — c = 299,792,458 m/s — that is identical for all observers (Einstein's special relativity), independent of the motion of the source or observer, and constant in time. Because c is precisely fixed: - 'One light-year' is a precisely defined distance. - Any astronomer anywhere using the same value of c gets the same distance. - The unit is independent of any local terrestrial reference. This makes the light-year a UNIVERSAL, STABLE, REPRODUCIBLE UNIT OF DISTANCE — perfect for cosmic scales. Why other options are WRONG: (A) Distances among stellar bodies DO CHANGE — galaxies move, stars orbit galactic centres, universe is expanding. Distances are not static. So this is not what makes the light-year useful. (B) Gravity of stellar bodies changes over time with their evolution (mass loss in winds, supernovae) and varies between bodies. It is not constant, and is irrelevant to the light-year definition. (C) Light DOES NOT always travel in a straight line — general relativity predicts (and observations confirm via gravitational lensing) that light's path is bent by gravitational fields near massive bodies. Light from distant quasars bends around galaxy clusters. So this is wrong physically too, and even if true, it would not make 'light-year' a useful unit. Source: NCERT Class 11 Physics 'Units and Measurements' / NCERT Class 12 Physics 'Wave Optics' / Class 11 Physics 'Gravitation'.