Explanation
SI unit of length is Metre, denoted by (m).
Angstrom is a unit of length and 1 Angstrom = $${{10}^{-10}}m$$
Fermi is a unit of length also known as femtometre and 1 fermi = $${{10}^{-15}}m$$
parsec is a unit of and 1 parsec = $$3.085\times {{10}^{16}}m$$
barn is unit of nuclear cross section, not length
$$\textbf{Answer}$$
$$\textbf{Hence, the correct option is (c) barn}$$
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