Which of the following is not unit of length?

Options

A. asgstrom
B. fermi
C. barn
D. parsec

Explanation

To solve the question "Which of the following is not a unit of length?" we need to analyze each of the given options:

1. **Option A: Angstrom**
- An angstrom (Å) is a unit of length commonly used in the field of atomic and molecular physics.
- It is defined as \(1 \, \text{Å} = 10^{-10} \, \text{meters}\).
- Therefore, angstrom is indeed a unit of length.

2. **Option B: Fermi**
- A fermi (fm) is another unit of length, primarily used in nuclear physics.
- It is defined as \(1 \, \text{fm} = 10^{-15} \, \text{meters}\).
- Thus, fermi is also a unit of length.

3. **Option C: Barn**
- A barn is a unit of area, not length.
- Specifically, \(1 \, \text{barn} = 10^{-28} \, \text{square meters}\).
- It is used to express cross-sectional areas in nuclear and particle physics, indicating that it is not a unit of length.

4. **Option D: Parsec**
- A parsec (pc) is a unit of length used in astronomy.
- It is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, and it is equivalent to approximately \(3.26 \, \text{light years}\).
- Therefore, parsec is also a unit of length.

After analyzing all the options, we conclude that the only option that is not a unit of length is:

**Answer: Option C: Barn**