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

The speed of light in a vacuum is a fundamental physical constant ($c \approx 3 \times 10^8$ m/s) that remains invariant regardless of the observer's motion, providing a stable and unchanging yardstick for measuring vast interstellar distances. While option (c) is a common misconception, light does not always travel in a straight line because it can be bent by the gravity of massive objects (gravitational lensing). The core concept being tested is the constancy of the speed of light as the basis for the light-year, which is a unit of distance, not time.