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

The alkene is methylenecyclobutane. Protonation of the exocyclic $\mathrm{CH}{}_2$ gives the tertiary 1-methylcyclobutyl cation, which is already the stable site, so there is no ring expansion. Bromide capture gives 1-bromo-1-methylcyclobutane.