The correct answer for why astronomical distances are measured in light-years is option 'D', which states that the speed of light is always the same. Here is a detailed explanation of this answer:

Understanding Light-years
- A light-year is a unit of measurement used in astronomy to represent the distance that light travels in one year.
- Light travels at a constant speed in a vacuum, which is approximately 299,792 kilometers per second or about 186,282 miles per second.

Reasons for Measuring in Light-years

1. Consistency of the Speed of Light
- One of the fundamental principles of physics is that the speed of light in a vacuum is constant and does not change.
- This means that light always travels at the same speed regardless of the distance it needs to cover.
- By using light-years as a unit of measurement, astronomers can account for this constant speed and accurately represent vast distances in space.

2. Vastness of Astronomical Distances
- The scale of astronomical distances is incredibly large, with objects in space being located millions or even billions of light-years away.
- Using other units of measurement, such as kilometers or miles, would result in very large and impractical numbers.
- Light-years provide a more convenient and understandable way to express these immense distances.

3. Time and Distance Relationship
- Since a light-year represents the distance light can travel in one year, it also indirectly gives us information about the time it takes for light to reach us from a distant object.
- For example, if we observe a star that is 10 light-years away, it means that the light we see from that star today actually started its journey 10 years ago.
- By using light-years, astronomers can easily relate time and distance, which is crucial for studying objects that are far away in space.

Conclusion
In conclusion, the reason why astronomical distances are measured in light-years is because the speed of light is constant and does not change. This consistency allows astronomers to accurately represent vast distances in space and relate them to the time it takes for light to travel. Using light-years as a unit of measurement provides a convenient and understandable way to express the immense scale of the universe.

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. 
So option (D) is correct.