Correct Option
The speed of light in a vacuum is a fundamental physical constant, universally accepted and precisely measured (approximately 299,792,458 meters per second). A light-year is defined as the distance light travels in one Julian year in a vacuum. Due to the constant and predictable nature of light's speed, it serves as a reliable and consistent unit for expressing the immense distances between celestial objects. This constancy ensures that a light-year represents a fixed and unambiguous measure of distance.
Incorrect Options
Option (A): Distances between stellar bodies are not constant. Celestial objects, including stars and galaxies, are in continuous motion, leading to changes in their relative positions and distances over time. Therefore, the premise that distances do not change is incorrect.
Option (B): The gravitational force exerted by stellar bodies is a fundamental property, but its constancy or variability does not directly influence the choice of a unit for measuring astronomical distances. While gravity affects the motion and structure of celestial objects, it is not the basis for defining a unit like the light-year.
Option (C): Light does not always travel in a perfectly straight line. Its path can be significantly altered by strong gravitational fields, a phenomenon known as gravitational lensing. This effect demonstrates that light's trajectory is not universally straight, making it an unsuitable basis for defining a distance unit in this context.