Which of the following is not a unit of length ?

Options

A. fermi
B. parsec
C. light year
D. poise

Answer

To determine which of the following is not a unit of length, we will analyze each option provided.

### Step-by-Step Solution:

1. **Identify the Units of Length**:
- Common units of length include meter (m), centimeter (cm), kilometer (km), millimeter (mm), light-year (ly), parsec (pc), and Fermi (fm).

2. **Evaluate Each Option**:
- **Fermi (fm)**: This is a unit of length equal to \(10^{-15}\) meters. It is indeed a unit of length.
- **Parsec (pc)**: This is a larger unit of length, approximately equal to 3.26 light-years. It is also a unit of length.
- **Light-year (ly)**: This is the distance that light travels in one year. It is a unit of length.
- **Poise**: This is a unit of dynamic viscosity, defined as \(1 \text{ poise} = 1 \frac{\text{g}}{\text{cm} \cdot \text{s}}\). It is not a unit of length.

3. **Conclusion**:
- Among the options, Poise is not a unit of length. Therefore, the answer is **D (Poise)**.