Which one is not a unit of time?

Options

A. Solar year
B. tropical year
C. leap year
D. light year

Explanation

To solve the question "Which one is not a unit of time?" we need to analyze the given options: solar year, tropical year, leap year, and light year.

### Step-by-Step Solution:

1. **Understanding the Options**:
- **Solar Year**: This is the time taken by the Earth to complete one orbit around the Sun, approximately 365.25 days. It is a unit of time.
- **Tropical Year**: This is the time taken for the Earth to return to the same position in the cycle of seasons, also about 365.24 days. It is also a unit of time.
- **Leap Year**: This is a year that has one extra day (February 29) added to keep the calendar year synchronized with the astronomical year. It is a unit of time as well.
- **Light Year**: This is the distance that light travels in one year, which is approximately 9.46 trillion kilometers. It is a unit of distance, not time.

2. **Identifying the Correct Answer**:
- Among the four options, solar year, tropical year, and leap year are all units of time. In contrast, light year is a unit of distance.

3. **Conclusion**:
- Therefore, the correct answer to the question "Which one is not a unit of time?" is **light year**.

### Final Answer:
**Light year** is not a unit of time.