The alkene 3,3-dimethylbut-1-ene, $(CH X_{3}) X_{3} C - CH = CH X_{2}$ , adds a proton at the terminal carbon to give a secondary cation, which then undergoes a single 1,2-shift to a more stable cation. The final cation is:

A2,3-dimethylbutan-2-yl cation, $(CH X_{3}) X_{2} C X^{+} - CH (CH X_{3}) X_{2}$
B3,3-dimethylbutyl cation, $(CH X_{3}) X_{3} C - CH X_{2} - CH X_{2} X^{+}$
C3,3-dimethylbutan-2-yl cation, $(CH X_{3}) X_{3} C - CH X^{+} - CH X_{3}$
D3-methylbutan-2-yl cation, $(CH X_{3}) X_{2} CH - CH X^{+} - CH X_{3}$

Answer: A

H+ adds to the CH2 end, giving

(CH X_{3}) X_{3} C - CH X^{+} - CH X_{3}

(secondary). A methyl group on the neighbouring quaternary carbon shifts with its bonding pair to the cationic carbon, leaving a tertiary cation

(CH X_{3}) X_{2} C X^{+} - CH (CH X_{3}) X_{2}

. Tertiary is more stable, so this is the final cation.