A = n-butyl bromide B = sec butyl bromide C = tert butyl bromide D = Iso butyl bromide order of reactivity towards ${\mathrm{SN}}^1$ mech is 

Rate of ${\mathrm{SN}}^1$ reactivity $\propto$ stability of carbo cation 

stability of carbocations $-3^o>2^o>1^o$

stability of ${\left({\mathrm{CH}}_3\right)}_2\mathrm{CH}\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2>{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{C}{\mathrm{H}}_2\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$ cannot be explained on the basis of H-conjugation. It can be explained on the basis of experimental facts & inductive effect

$S{N}^1$ reactivity ${\left(C{H}_3\right)}_{3}CBr>C{H}_{3}CHBrC{H}_{2}C{H}_3>{\left(C{H}_3\right)}_{2}CHC{H}_{2}Br>C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}Br$

$\therefore C>B>D>A$