Atomic mass unit is abbreviated as ______________.
A) atm
B) ama
C) a.m.u
D) aum

Hint: The mass of an atom of an element is known as its atomic mass. Since it has a very small value, hence, it is not feasible to express it in grams or kilograms. So, IUPAC discovered new units for them.

Complete step-by-step answer:
The unit atm represents the atmosphere, which is the unit of pressure. The units ama and aum are not associated with any physical quantity in chemistry.

The Atomic Mass Unit, abbreviated as a.m.u. is defined as precisely $\dfrac{1}{{12}}$the mass of an atom of C-12 isotope. The C-12 isotope has 6 protons and 6 neutrons in its nucleus.

C-12 isotope has been chosen as the standard element. The masses of all other elements are calculated with respect to the $\dfrac{1}{{12}}$ of the C-12 isotope.

In other terms, it can also be said that the atomic mass unit is the average of the mass of a proton at rest and the mass of a neutron at rest.

Its value is approximately equal to $1.67377 \times {10^{ - 27}}kg$or $1.67377 \times {10^{ - 24}}g$.
So, if we write that the Atomic Mass of Sodium Atom is\[23{\text{ }}a.m.u.\], then we mean to say that its actual mass is $23 \times 1.67377 \times {10^{ - 24}}g$.

Hence the correct answer is option ‘C’.

Note: Students should take care of the fact that the mass number and the atomic mass of an element are not one and the same thing. Mass Number refers to the sum of the number of protons and neutrons present inside the nucleus of an atom whereas Atomic Mass refers to the actual mass of the atom.