Introduction: Carbocation is a positively charged carbon atom that is highly unstable due to the absence of an octet of electrons. The stability of carbocation depends on the type of alkyl group attached to the positively charged carbon atom.

Explanation:

a) PhCH2: This carbocation is attached to a phenyl group, which is an electron-donating group. Hence, the positive charge on the carbon atom is stabilized by the delocalization of electrons into the phenyl ring. Thus, it is the most stable carbocation among the given options.

b) CH=CH2: This carbocation is attached to a vinyl group, which is an electron-withdrawing group. The positive charge on the carbon atom is destabilized by the double bond, which is electron-rich. Thus, it is the least stable carbocation among the given options.

c) Me2CH2: This carbocation is attached to two methyl groups, which are electron-donating groups. The positive charge on the carbon atom is stabilized by the hyperconjugation effect of the two methyl groups. Thus, it is more stable than CH=CH2 but less stable than PhCH2.

d) CH2=CH-CH2: This carbocation is attached to an allyl group, which is an electron-donating group. The positive charge on the carbon atom is stabilized by the resonance effect of the double bond and the allyl group. Thus, it is more stable than CH=CH2 but less stable than PhCH2.

Conclusion: The least stable carbocation among the given options is CH=CH2, while the most stable carbocation is PhCH2. The stability of carbocation depends on the type of alkyl group attached to the positively charged carbon atom.

B