The rate of ${\mathrm{S}}_{\mathrm{N}}1$ reaction $\propto$ stability of carbocation
stability of carbocation $\propto +\mathrm{v}\mathrm{e}$ charge delocation.
$\propto +\mathrm{M},\mathrm{ }+\mathrm{H}\mathrm{C},\mathrm{ }+\mathrm{I}$
Order of effect effectiveness
$+\mathrm{M}>+\mathrm{H}\mathrm{C}>+\mathrm{I}\mathrm{ }(+\mathrm{M}\mathrm{ })>\mathrm{ }(-\mathrm{I})$
D 

C 

A 

B 
So, order of stability $\mathrm{B}<\mathrm{A}<\mathrm{C}<\mathrm{D}$