Which of the following is not a unit of length / distance ?

Options

A. Picometer
B. Light year
C. Kilometre
D. Radian

Answer

To solve the question "Which of the following is not a unit of length/distance?", we will analyze each of the given options to determine if they are units of length or not.

### Step-by-Step Solution:

1. **Identify the Options**: The options provided are:
- A) Picometer
- B) Light Year
- C) Kilometer
- D) Radian

2. **Analyze Each Option**:
- **A) Picometer**:
- A picometer is a unit of length in the metric system, equal to \(10^{-12}\) meters.
- **Conclusion**: This is a unit of length.

- **B) Light Year**:
- A light year is the distance that light travels in one year. It is commonly used in astronomy to measure vast distances in space.
- **Conclusion**: This is a unit of length.

- **C) Kilometer**:
- A kilometer is a metric unit of length equal to \(10^3\) meters.
- **Conclusion**: This is a unit of length.

- **D) Radian**:
- A radian is a unit of angular measure. It is used to measure angles, not distances.
- **Conclusion**: This is NOT a unit of length.

3. **Final Answer**:
- Based on the analysis, the option that is NOT a unit of length or distance is **D) Radian**.