Explanation:
Consider, \({\rm{n = }}\) number of neutrons
\({\rm{(1)}}\,\,\,\,_{\rm{2}}^{\rm{4}}{\rm{He}} \Rightarrow {\rm{n = 4 - 2 = 2}}\)
\({\rm{(2)}}\,\,\,\,_{\rm{3}}^{\rm{7}}{\rm{Li}} \Rightarrow {\rm{n = 7 - 3 = 4}}\)
\({\rm{(3)}}\,\,\,\,_{\rm{1}}^{\rm{1}}{\rm{H}} \Rightarrow {\rm{n = 1 - 1 = 0}}\)
\({\rm{(4)}}\,\,\,_{\rm{1}}^{\rm{3}}{\rm{H}} \Rightarrow {\rm{n = 3 - 1 = 2}}\)