The alkene shown is treated with $\mathrm{HBr}$ under radical conditions ($\mathrm{ROOR}$). Which structure is the major product?

The alkene is styrene. $\mathrm{Br}$ radical adds to the terminal $\mathrm{CH}{}_2$ carbon because this gives the benzylic radical, which is resonance stabilised; hydrogen abstraction from $\mathrm{HBr}$ completes the step. The product is (2-bromoethyl)benzene (anti-Markovnikov); without peroxide the ionic route would give (1-bromoethyl)benzene.