The most reactive amine towards reactions with dil. HCl is

More be the ability of $N$-atom of $- {NH}_{2}$ group to donate lone pair of electrons to $HCl$ (dil.), more be the reactivity of amine.

In (B) two alkyl groups are attached to the $- {NH}_{2}$ (having electron releasing nature) which increases the electron donating ability of $N$-atom of $- {NH}_{2}$. 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.