Question

Which of the following is not the unit of length.
(A) angstrom
(B) fermi
(C) barn
(D) parsec

Answer 1

To measure a physical quantity we require a standard of measurement. This standard is called the unit of that physical quantity. The measure of any physical quantity is equal to $nu$
Where, $n$ = numerical value of the measure of the quantity and $u$ = unit of the quantity.

Complete step by step solution:
The SI (International system of units) unit of length is Metre. One metre is defined as distance travelled by light in vacuum in $\dfrac{1}{{299,792,458}}$ second.
Now we have to find out among the given units which is not the unit of length. We will discuss each in detail.
angstrom is a unit of length. Though angstrom is not a part of the SI system of units, it can be considered part of the metric system in general. The unit is still often used in the natural sciences and technology to express sizes of atoms, molecules, microscopic biological structures, and lengths of chemical bonds, arrangement of atoms in crystals, wavelengths of electromagnetic radiation, and dimensions of integrated circuit parts.
fermi is also the unit of length. A fermi is equivalent to a femtometer, and is the older non SI measurement unit of length, in honor of Italian physicist Enrico Fermi, since it is a typical length-scale of, and often referenced in nuclear physics.
a barn is a metric unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions.
parsec is a unit of length used to measure the large distances to astronomical objects outside the Solar System.

Hence from above we can say that option (C) is correct.

Note: Basically we have seven fundamental quantities in physics and the units defined for fundamental quantities are called fundamental units. Beside the seven fundamental units we have two supplementary units. They are for plane and solid angle.