a)Aismorestablebecauseitisaterminalcarbocation,accordingtomarkonikov′srule.b)Aismorestablebecauseitisstabilizedbyresonanceandhyperconjugation.c)Aismorestablebecausecationiccarbonissphybridized.d)Bismorestablebecauseithas2alkylgroupsthatcanprovideextrastabilizationbyhyperconjugationandinductiveeffect.