In which of the following reactions is the major product formed with a rearranged carbon skeleton (compared with the starting alkene)?

Free carbocations rearrange. 3,3-Dimethylbut-1-ene with HCl forms a secondary cation, then a methyl shift gives the tertiary cation and 2-chloro-2,3-dimethylbutane. 3-Methylbut-1-ene on acid hydration undergoes a hydride shift to give 2-methylbutan-2-ol. Oxymercuration and hydroboration involve no free carbocation, so the skeleton is retained.