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

[A] Distances among stellar bodies do not change.

[B] Gravity of stellar bodies does not change.

[C] Light always travels in straight line.

[D] Speed of light is always same.

Answer: D
Notes:

Why this Question) Important basic concept, Indian Space Missions, Gaganyaan and others. 

 

Ans) d 

Exp) Option d is correct. 

A light-year is a measurement of distance in space. The astronomical distances are measured in light-years because, the speed of light is constant throughout the universe and is known to high precision. In a vacuum, light travels at 670,616,629 mph (1,079,252,849 km/h). To find the distance of a light-year, you multiply this speed by the number of hours in a year (8,766). The result: One light-year equals 5,878,625,370,000 miles (9.5 trillion km). At first glance, this may seem like an extreme distance, but the enormous scale of the universe dwarfs this length.  

Measuring in light-years also allows astronomers to determine how far back in time they are viewing. Because light takes time to travel to our eyes, everything we view in the night sky has already happened. In other words, when you observe something 1 light-year away, you see it as it appeared exactly one year ago. 

Source) https://www.space.com/light-year.html  

 Subject) Science