**Explanation:**

The correct answer is option **B) Distance**.

A light-year is a unit of distance used in astronomy to measure large distances in space. It is defined as the distance that light travels in one year in a vacuum. Since light travels at a constant speed of approximately 299,792 kilometers per second (or about 186,282 miles per second), the distance covered by light in one year is immense.

To better understand the concept of a light-year, let's break down the distance calculation and the significance of using this unit in astronomy:

**Definition of a Light-Year:**
- A light-year is the distance that light travels in one year.
- Light travels at a speed of about 299,792 kilometers per second (or about 186,282 miles per second).
- Therefore, in one year, light can travel about 9.46 trillion kilometers (or about 5.88 trillion miles).

**Importance of Light-Years in Astronomy:**
- The vast distances between celestial objects in space make it impractical to use conventional units like kilometers or miles.
- Astronomers use light-years to measure the distances between stars, galaxies, and other objects in the universe.
- Light-years allow us to comprehend the enormous scale of the universe and the time it takes for light to travel across such vast distances.

**Examples of Light-Years:**
- The nearest star to Earth, Proxima Centauri, is approximately 4.24 light-years away.
- The Andromeda Galaxy, our closest neighboring galaxy, is about 2.537 million light-years away.
- The observable universe is estimated to be about 93 billion light-years in diameter.

In conclusion, a light-year is a unit of distance used in astronomy to measure the vast distances between celestial objects. It represents the distance that light travels in one year and is an essential tool for understanding the scale and size of the universe.