$\begin{array}{1 1}a)\;A\; is\; more\; stable\; because\; it is\; a\; terminal \;carbocation,according\; to\; markonikov's\; rule.\\b)\; A \;is\; more\; stable\; because\; it\; is\; stabilized\; by\; resonance \;and\;hyper conjugation.\\c)\; A\; is\; more\; stable\; because \;cationic\; carbon\; is\; sphybridized.\\d)\; B\; is\; more \;stable\; because\; it\; has \;2\; alkyl\; groups \;that\; can\;provide \;extra \;stabilization\; by\; hyperconjugation \;and \;inductive\;effect.\end{array}$