Select the cation(s) which cannot be converted into a more stable carbocation by any single 1,2-hydride or 1,2-methyl shift.

A2-methylpropan-2-yl cation, $(CH X_{3}) X_{3} C X^{+}$
B3-methylbutan-2-yl cation, $CH X_{3} - CH X^{+} - CH (CH X_{3}) X_{2}$
C2,3-dimethylbutan-2-yl cation, $(CH X_{3}) X_{2} C X^{+} - CH (CH X_{3}) X_{2}$
D2,2-dimethylpropyl cation, $(CH X_{3}) X_{3} C - CH X_{2} X^{+}$

Answer: A, C

Tertiary cations (first two) can only shift to give a secondary or primary cation, so they stay. The 3-methylbutan-2-yl cation (

2^{\circ}

) gives a

3^{\circ}

cation by a hydride shift, and the primary neopentyl cation gives a

3^{\circ}

cation by a methyl shift.