The alkene shown reacts with $\mathrm{HBr}$. Which structure is the major product?

The alkene is 3,3-dimethylbut-1-ene. The secondary cation formed on protonation is next to a quaternary carbon, so a methyl group shifts with its bonding pair to give a tertiary cation. Bromide capture gives 2-bromo-2,3-dimethylbutane as the major product.