\(\underbrace{\stackrel{1}{C} H_{2}=\;\stackrel{...2}CH-\stackrel{...3}C}_{\stackrel{\oplus}I} H \leftarrow C H_{3}\)

\(C H_{3} \rightarrow \underbrace{\stackrel{3}C H-\stackrel{2}C H-\stackrel{1}C H_{2}}_{\stackrel{\oplus}{I I I}}\)

We know that better the dispersal of \(+\) charge, more will be the stability of the carbonium ion. Further, we know that \(C_1\) and \(C_3\) carry most of the positive charge which is

\(\stackrel{1}{C} H_{2}=\stackrel{2}{C} H-{\stackrel{3}{C}} \;^\oplus H \leftarrow C H_{3} \leftrightarrow\) \(\stackrel{1}{C}\,^\oplus H_{2}-\stackrel{2}C H=\stackrel{3}C H-C H_{3}\)

dispersed by the methyl group ( \(+ I\) group) present on \(I\) and \(II\), thus these two are more and equally stable than the \(II\) in which methyl group is present on \(C_2\) which carry little of the positive charge.