Length cannot be measured by:
A: Fermi
B: Debye
C: Micron
D: Light year

Hint: We’re given four units out of which we have to choose the unit that doesn’t measure length. For that, we have to go through the definitions of each unit and infer what it symbolizes. This will make it easier to arrive at a conclusion.

Complete Step by step answer: We know that Fermi is the unit of length where 1 Fermi is equal to \[{{10}^{-15}}\]metres. It is similar to the value of the diameter of a proton. It measures distances in atomic units.
Considering Debye, we can say that it is the unit which helps to express the electric dipole moments that exist between molecules. The value of one Debye is equal to $3.336\times {{10}^{-30}}Cm$ (Coulomb metre). It is also considered as the unit of force in the CGS system.
Our next option is micron, which is the unit of length that is equal to one millionth of a metre. It is used to measure the tiniest of all quantities. One micron is ${{10}^{-6}}m$ and it is used in various scientific and technological fields.
The last option is light year, which is a unit of astronomical distance which is equal to the total distance travelled by light in a year, that is $9.4607\times {{10}^{12}} km$.
From the definitions of the four options, we can arrive at the conclusion that option B, that is Debye is a unit of force whereas the other three quantities are units of length.

Therefore, option B is the correct answer among the given options.

Note: Light year is often confused by students as a unit of time as it emphasises the word year. However, it measures the distance travelled by light in a year. It is the unit of length. This should be kept in mind.