Answer: Option (b) is the correct answer.
Step-by-step explanation:
An atom consists of three subatomic particles, that is, protons, neutrons, and electrons.
Protons and neutrons reside inside the nucleus of an atom whereas electrons revolve around the nucleus of the atom.
Since protons are positively charged and neutrons have no charge so, due to the like charges of protons they tend to repel each other through electrostatic force of repulsion. But the energy which binds both protons and neutrons together in a nucleus is known as nuclear binding energy.
This energy only balances the nucleus when there are almost similar number of protons and neutrons. But when there is great difference in the number of protons and neutrons then the atom becomes unstable.
Thus, we can conclude that some atoms become unstable because neutrons and protons are off-balance.