Which among the following would undergo S_N² reaction faster?

S_N² mechanism involves simultaneous breaking and formation of bond between the nucleophile and the reactant. This step involves attack of nucleophile from that side which gives less stearic hindrance for the approach of nucleophile.

In case of isobutyl bromide, there is a lot of stearic hindrance due to presence of 3 methyl groups along the carbon atom. Thus only bromoethane can undergo S_N² reaction. As a general we remember this, less substituted alkanes can undergo S_N² reaction faster due to less stearic hindrance