The alkene shown reacts with $\mathrm{HBr}$ (no peroxide). Which structure is the major product?

The alkene is (prop-1-en-2-yl)cyclopentane. Protonation of the terminal $\mathrm{CH}{}_2$ carbon gives a tertiary carbocation directly. A ring expansion would convert it into a secondary cyclohexyl cation, which is less stable, and the cyclopentane ring has little angle strain to relieve, so no rearrangement occurs. Bromide captures the tertiary cation to give (2-bromopropan-2-yl)cyclopentane, Structure C.