Which of the following is not a unit of time?

Options

A. Parsec
B. Year
C. second
D. Hour

Explanation

To determine which of the following options is not a unit of time, we will analyze each option provided:

1. **Option A: Parsec**
- A parsec is a unit of distance used in astronomy. It is defined as the distance at which one astronomical unit subtends an angle of one arcsecond. Specifically, one parsec is approximately equal to 3.26 light-years. Since it measures distance, it is not a unit of time.

2. **Option B: Year**
- A year is a unit of time that represents the duration it takes for the Earth to complete one orbit around the Sun. It is commonly used to measure longer periods of time. Therefore, a year is indeed a unit of time.

3. **Option C: Second**
- The second is the base unit of time in the International System of Units (SI). It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. Hence, the second is a unit of time.

4. **Option D: Hour**
- An hour is another unit of time, which is equal to 60 minutes or 3600 seconds. It is widely used in everyday life to measure time intervals. Thus, an hour is also a unit of time.

After analyzing all the options, we conclude that:

- **Correct Answer:** Option A: Parsec is not a unit of time.

### Summary of Steps:
1. Identify and analyze each option.
2. Determine if each option is a unit of time or not.
3. Conclude which option is not a unit of time.