Answer:
D. A nucleus of 2 protons and 2 neutrons.
Step-by-step explanation:
A stable nucleus is one that do not disintegrate naturally, majorly due to the even number of its protons and neutrons. Thus they are not radioactive in nature.
From the given question, it would be observed that a nucleus of 2 protons and 2 neutrons would give a helium atom. And this is a stable atoms due to the magnetic number of the components of its nucleus.
Thus, the nuclear configuration that is most stable out of the given options is a nucleus of 2 protons and 2 neutrons. Option D.