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

The correct answer is D. A light-year is the distance light covers in vacuum in one Julian year, about 9.46 x 10^12 km. It works as a yardstick only because the speed of light in vacuum is a fixed constant, c = 3 x 10^8 m/s, independent of the motion of the source or observer. This is a central postulate of Einstein's special relativity. Since the ruler does not stretch, distances expressed in light-years stay comparable across the universe.

Why the others fail:

• A: Stars and galaxies do move, and cosmic expansion changes distances over time. That has nothing to do with choosing a unit.
• B: Gravity of a stellar body depends on its mass and distance, so it varies. It is not the reason for a unit of length.
• C: Light bends in a gravitational field (gravitational lensing, predicted by general relativity), so it does not always travel in a straight line.

Concept: invariance of c in vacuum. Mains angle: the cosmic distance ladder, and why parallax, standard candles and redshift are needed to map the universe (GS3, science and technology). Light slows in media like water or glass, but the light-year is defined in vacuum, so that does not weaken the logic.

Trap: Many aspirants pick A because stars look fixed to the naked eye. Stars do move, but the unit does not depend on that.