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

The alkene is 3-methylbut-1-ene. Protonation gives a secondary cation at C-2, which undergoes a 1,2-hydride shift from C-3 to give the more stable tertiary cation. Chloride then adds to give 2-chloro-2-methylbutane.