Explanation

(b) The given reaction is a nuclear reaction, which can take place only if a proton (a hydrogen nucleus) comes into contact with a lithium nucleus. If the hydrogen is in the atomic from, the interaction between it’s electron cloud and the electron cloud of a lithium atom keeps the two nuclei from getting close to each other. Even if isolated protons are used, they must be fired at the $Li$ atom with enough kinetic energy to overcome the electric repulsion between the proton and $Li$ atom