Assertion : Protons carrying positive charges are compactly residing inside the nucleus.

Reason : Electrostatic repulsive force between protons is very weak.

There is a third kind of fundamental force called strong force which holds the protons together inside the nucleus. It is very large compared to that of the repulsion forces between protons, Its range is about ${10}^{-14} m$ and this is precisely the size of the nucleus.

Both assertion and the reason are true but reason is not the correct explanation of assertion.