Final answer:
The sec-butyl cation is the most stable carbocation among the given options due to it being a secondary carbocation with greater stabilizing effects such as hyperconjugation and inductive effects compared to the isobutyl and n-butyl cations.
Step-by-step explanation:
The stability of carbocations increases in the order of primary (RCH₂) < secondary (R₂CH) < tertiary (R₃C). Therefore, between the sec-butyl cation, the isobutyl cation, and the n-butyl cation, the most stable is the sec-butyl cation.
This is because the sec-butyl cation is a secondary carbocation, whereas the n-butyl cation is a primary carbocation, and stability is less for primary carbocations. The isobutyl group is less stable due to less hyperconjugation when compared to the sec-butyl cation.
The mentioned correct option in the final answer is the sec-butyl cation, which is the most stable out of the options given.