More be the ability of $\mathrm{N}$-atom of $-{\mathrm{NH}}_2$ group to donate lone pair of electrons to $\mathrm{HCl}$ (dil.), more be the reactivity of amine.

In (B) two alkyl groups are attached to the $-{\mathrm{NH}}_2$ (having electron releasing nature) which increases the electron donating ability of $\mathrm{N}$-atom of $-{\mathrm{NH}}_2$. In aqueous medium, secondary amine is most basic. Hence, it is most reactive.  In (D), due to resonance, lone pair of electrons get delocalised and its basic strength is very less.