Which of the following alkyl halide is most reactive towards solvolysis?  

In the following reaction the most probable product will be:

In ${\mathrm{S}}_{\mathrm{N}}2$ substitution reaction :-

$\mathrm{R}-\mathrm{Br}+\mathrm{Nal}\stackrel{\mathrm{Acetone}}{\to }\mathrm{R}-\mathrm{l}+\mathrm{NaBr}$

Which one of the following has the highest relative rate?