If Z is the number of protons and A the number of nucleons, then the number of neutrons in an atom is given by

The correct option is B

A - Z

Given,
Z is the number of protons and A is the number of nucleons (electrons + protons) in an atom.
Therefore, number of neutrons in the atom is given by A-Z.

For example:
Atomic number (Z) of an oxygen atom is 6. Which means the atom has 6 protons.
Mass number (A) of this atom is 12, which means it has 6 protons and 6 neutrons in its nucleus.

Therefore, the number of neutrons in the atom is given by :
A-Z = 12 - 6
A-Z = 6