a
\({S_{{N^1}}}\) reactions involve the formation of carboations, so greater the stability of the carbocation formed by alkylhalide more will be its reactivity toward \({S_{{N^1}}}\) reaction.

\({{C}_{6}}{{H}_{5}}\overset{+}{\mathop{C}}\,{{H}_{2}}>C{{H}_{2}}=CH\overset{+}{\mathop{C}}\,{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{2}}\overset{+}{\mathop{C}}\,H>C{{H}_{3}}\overset{+}{\mathop{C}}\,{{H}_{2}}\)