Given: Nuclear reaction: ${ }_{1} H ^{2}+{ }_{1} H ^{2} \rightarrow{ }_{2} He ^{3}+?$.
We know that the balanced equation of the nuclear reaction is ${ }_{1} H ^{2}+{ }_{1} H ^{2} \rightarrow{ }_{2} He ^{3}+{ }_{0} n ^{1}$. Where ${ }_{0} n ^{1}$ is the symbol of neutron.
Therefore neutron will be added in the given equation to complete it.