Addition of HBr to isobutylene mainly gives

Explanation

The addition of HBr to isobutylene predominantly yields tert-butyl bromide because the reaction proceeds through a carbocation intermediate. When HBr adds to isobutylene, the double bond opens up, and the bromine atom attaches to the more substituted carbon, leading to the formation of a tertiary carbocation. This intermediate is more stable than secondary or primary carbocations, favoring the formation of tert-butyl bromide. In contrast, isobutyl bromide, n-butyl bromide, and sec-butyl bromide would require pathways that do not occur under these conditions, making them incorrect options.