d
Stability of carbonium ion or carbocation depends on the adjacent alkyl groups. Alkyl groups exhibit electron releasing \(+ I\) effect and hyperconjugative effect which stabilizes the positive charge. \(CH _3^{+}\) methyl carbonium ion does not have both of these effects, therefore it is the least stable.

Therefore \(\begin{array}{*{20}{c}} {\,\,\,\,\,C{H_3}} \\ | \\ {\mathop {C{H_3} - C - C{H_3}}\limits_ +  } \end{array}\) have maximum stability.