The secondary bromide shown solvolyses slowly in 80 percent aqueous ethanol at 25 degrees C. Which of the structures A to D solvolyses fastest under the same conditions?

The reference is 2-bromopropane. $\mathrm{S}{}_{\mathrm{N}}1$ solvolysis rate tracks carbocation stability and ability to ionise. 2-Bromo-2-methylpropane gives a tertiary cation and is fastest. The bridgehead bromide is tertiary but cannot flatten to a planar cation, the neopentyl bromide would need a primary cation (or a shift), and bromobenzene gives an unstable aryl cation.