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

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

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.