($d$)is most stable and compound ($a$) is least stable among these in compound ($b$) and ($c$) carbon atom of that positive charge is $\mathrm{sp}^3$ hybridised they on the basis of angle strain theory compound ($c$) is more stable than compound ($b$).
$(i)$ $\begin{array}{*{20}{c}}
{\begin{array}{*{20}{c}}
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C{H_3}-CH_2} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,|}
\end{array}} \\
{C{H_3} - {C^ + }} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C{H_3}}
\end{array}$
$(ii)$ $\begin{array}{*{20}{c}}
{\begin{array}{*{20}{c}}
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C{H_3}} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,|}
\end{array}} \\
{C{H_3} - {C^ + }} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{H}}
\end{array}$
$(iii)$ $\begin{array}{*{20}{c}}
{\begin{array}{*{20}{c}}
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C{H_3}} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,|}
\end{array}} \\
{C{H_3} - {C^ + }} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,|} \\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C{H_3}}
\end{array}$
$(iv)$ $Ph - CH ^{+}-\underline{ CH }_{3}$
$[Nu^-=(A) \,PhO^-, (B)\,AcO^-, (C)\, HO^-, (D)\, CH_3O^-]$
$CH_3 - CH_3, CH_3 - CH_2 $$- CH_3, (CH_3)_2CH - CH_3,$
$ CH_3 - CH_2 - CH(CH_3)_2.$
આ મૂલકોનો સ્થાયિતાનો વધતો ક્રમ કયો છે?