Correct option is C. Forces other than the strong nuclear force make the lithium nucleus less stable
All neutrons attract each other with the same strong nuclear force. So, the strong nuclear force holds together three protons and one neutron $$\left( _{ 3 }^{ 4 }{ Li } \right) $$ just as vigorously as it holds together two protons and two neutrons $$\left( _{ 2 }^{ 4 }{ He } \right) $$. Specifically, protons repel other protons due to the positive charge over them. This repulsion tries to make a nucleus fly apart. Since $$\left( _{ 2 }^{ 4 }{ He } \right) $$ contains only two protons, the attractive strong nuclear forces overcome the repulsion of the protons. Hence, the nucleus holds together. But in $$\left( _{ 3 }^{ 4 }{ Li } \right) $$, the mutual repulsion of the three protons overcomes the strong nuclear attractions and the nucleus falls apart (or undergoes radioactive decay into a more stable nucleus). Therefore, the answer will be (c)