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

The correct answer is Option 4: Speed of light is always same.

In the vast expanse of the universe, conventional units like kilometers are inadequate due to the immense scales involved. Astronomical distances are measured in light-years because the speed of light is a universal constant (approximately 299,792,458 meters per second in a vacuum). This constancy provides a reliable and unchanging yardstick for deep-space measurements.

• Option 1 is incorrect: Stellar bodies are in constant motion relative to each other due to galactic rotation and cosmic expansion.
• Option 2 is incorrect: Gravity varies based on mass and distance, and is not a unit of spatial measurement.
• Option 3 is incorrect: While light travels straight in a vacuum, it can be bent by massive objects (gravitational lensing).

Therefore, Option 4 is the only scientifically accurate reason; since the speed of light never changes, the distance it travels in one year remains a fixed, absolute value.