Which of the following units can possibly be a unit of time?

Options

A. Leap year
B. Tropical year
C. Sidereal year
D. Light year

Answer

To determine which of the given options can be a unit of time, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Understanding Leap Year**:
- A leap year occurs every four years and has 366 days, which includes an extra day (February 29).
- Since a year is a measure of time, a leap year can indeed be considered a unit of time.
- **Conclusion**: Leap year is a unit of time.

2. **Understanding Tropical Year**:
- A tropical year is defined as the time it takes for the Earth to complete one orbit around the Sun, returning to the same position in the cycle of seasons.
- This period is approximately 365.24 days.
- Since it is based on the Earth's orbit and is a measure of time, the tropical year is also a unit of time.
- **Conclusion**: Tropical year is a unit of time.

3. **Understanding Sidereal Year**:
- A sidereal year is the time it takes for the Earth to complete one orbit around the Sun relative to the fixed stars.
- This period is about 365 days and 6 hours.
- Like the tropical year, it is also a measure of time, making the sidereal year a unit of time as well.
- **Conclusion**: Sidereal year is a unit of time.

4. **Understanding Light Year**:
- A light year is defined as the distance that light travels in one year, which is approximately 9.46 x 10^12 kilometers.
- It is not a measure of time but rather a measure of distance.
- **Conclusion**: Light year is NOT a unit of time.

### Final Answer:
The units that can possibly be a unit of time are:
- Leap Year
- Tropical Year
- Sidereal Year

The unit that is NOT a unit of time is:
- Light Year