Final answer:
Benzene can be protonated to form a resonance-stabilized carbocation due to its delocalized electrons. Ethene, propene, and butene do not have a structure that supports such stabilization.
Step-by-step explanation:
The question asks which alkene can be protonated to directly afford a resonance-stabilized carbocation. Among ethene, propene, butene, and benzene, the correct answer is D) Benzene. Although benzene is not an alkene but an aromatic hydrocarbon, its unique structure allows it to form a resonance-stabilized carbocation upon protonation due to the delocalized electrons in its ring structure, which are not present in the other alkenes listed. Ethene, propene, and butene do not provide the same level of stabilization for a carbocation through resonance.