The mass number of an atom is slightly less than the actual atomic mass.

In a neutral atom, the number of protons = the number of electrons.

The mass number represents the total number of protons and neutrons present in the nucleus. The actual atomic mass is calculated by taking into account all the isotopes and their relative quantities. Elements from nature exist as more than one isotope. For example, carbon has three isotopes,

Isotopes have an atomic weight of 12,13,14 and a relative mass of $98.892\%1.108\%, 2 \times {10}^{-10}\%$, respectively.

Thus the average atomic mass of the carbon atom will be$\mathrm{Avg}. \mathrm{Atomic} \mathrm{mass}= (0.988920) \left(12\right) + (0.01108) (13.0033) + (2\times {10}^{-10}) (14.00317)$$= 12.011\mathrm{u}$

Now, the atomic number of the carbon is 6 which indicates the number of protons is 6 and the mass number of carbon is 12 as the number of neutrons is 6, and the number of protons 6.

Hence, The mass number of an atom is slightly less than the actual atomic mass.

Thus, the given statement is TRUE.

Hence, option A is correct.