In which of the following reactions does $NH_3$ acts as an acid?

Question

(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.