ANS:C - Barn

Among the options provided, barn is not a unit of length.

1. Parsec (pc): A unit of length used in astronomy, equal to about 3.26 light-years, approximately 30.857 trillion kilometers, or about 19.173 trillion miles.
2. Angstrom (Å): A unit of length used to measure atomic and molecular distances, equal to 0.1 nanometers or 10−1010−10 meters.
3. Barn (b): A unit of area used in nuclear physics to measure the cross-sectional area of atomic nuclei or nuclear reactions. It is equivalent to 10−2810−28 square meters.
4. Fermi (fm): A unit of length used in nuclear physics to measure distances between atomic nuclei or nuclear particles, equal to 10−1510−15 meters or one femtometer.