(a) $NH_3 + HCl \longrightarrow NH_4Cl$
(b) $NH_3 + H^+ \longrightarrow NH^+_4$
(c) $NH_3 + Na \longrightarrow NaNH_2 + \frac{1}{2}H_2$
(d) $NH_3$ cannot act as an acid.
