Question

Which of the following is the smallest unit of distance?
A. Millimeter
B. Angstrom
C. Fermi
D. Meter

Answer 1

The instruments used for measurements of units like angstrom and Fermi are very big and costly and are found only in laboratories and research institutes. This is because the size of units like Fermi and angstrom is so small that it is very hard to measure them. Sophisticated instruments like mass spectrometers are used to measure these units.

Complete step by step answer:
All of the above mentioned options are units of distance. The main difference between them is that millimeters and meters are measurement scale units and are visible to the human eye.
$1{\text{ millimeter = }}{10^{ - 3}}{\text{meter}}$
And on the other hand, Angstrom and Fermi are used for the measurement of atomic and subatomic particles. These measurement units are not visible to the human eye. Therefore, one can easily eliminate millimeter and meter from the above given options.
$1{\text{ angstrom = }}{10^{ - 10}}{\text{meter}}$
$1{\text{ fermi = }}{10^{ - 15}}{\text{meter}}$
Angstrom is used for measuring the wavelengths of light or electro-magnetic waves. 1 angstrom can also be written as${10^{ - 10}}{\text{centi - meter}}$. The diameter of an atom is given by 1 angstrom. Also, the nucleus has the diameter of${10^{ - 13}}{\text{centi - meter}}$. This unit is mostly used by Atomic physicists.
The unit Fermi was derived by Enrico Fermi and it is used to deal with subatomic particles. Particles that are even smaller than the nucleus like protons and neutrons also (known as nucleons) have this unit. This unit is mostly used by Particle physicists.

So, the correct answer is “Option C”.

Note:
In the options given, Fermi is the smallest unit of distance.
But actually, the Planks length is the smallest possible size ever found in science. It is approximately $1.6 \times {10^{ - 35}}meter$.
Planks length is the distance the light travels in a perfect vacuum within the interval of Planks unit of time.