The mass number of a nucleus is equal to the number of

The correct answer is Option (4) → nucleons it contains

The mass number A is the total number of protons and neutrons in a nucleus, \( A=Z+N \). Protons and neutrons are both called nucleons, so A is the number of nucleons. The electrons lie outside the nucleus and are not counted.

NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “The composition of a nucleus can now be described using the following terms and symbols:

    \( Z \) – atomic number = number of protons \( \qquad [13.4(a)] \)

    \( N \) – neutron number = number of neutrons \( \qquad [13.4(b)] \)

    \( A \) – mass number = \( Z+N \) = total number of protons and neutrons \( \qquad [13.4(c)] \)

One also uses the term nucleon for a proton or a neutron. Thus the number of nucleons in an atom is its mass number A.”