(a) Distances among stellar bodies do not change. 

(b) Gravity of stellar bodies does not change. 

(c) Light always travels in a straight line. 

(d) Speed of light is always the same. 

Ans: d 

Explanation:

Light-year is the distance that light travels in one year. Light travels through interstellar space at 300,000 kilometers per second and 9.46 trillion kilometers per year in vacuum. The basic postulate of the ‘theory of relativity’ is that the speed of light is same in all inertial frames. This can be broken down into two parts:

• The speed of light is independent of the motion of the observer.
• The speed of light does not vary with time or place.

The distances between the stellar bodies are very large. These  bodies too are moving apart continuosly as the Universe is expanding. Therefore these above stated properties of light are the reasons that light is used to measure such large interstellar distances in light years.

Hence, option d is correct.

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