b
Ammonia acts as a ligand as it donates its lone pair of electrons to \(C u^{2+}\) ions to form \(\left[ Cu \left( NH _{3}\right)_{4}\right]^{2+}\) complex. This is possible in basic medium. In acidic medium, the lone pair of electrons present on ammonia is donated to proton to form ammonium ion which is not a ligand and cannot form a complex with \(C u^{2+}\) ions.

\(C u^{2+}(a q)+4 N H_{3} \stackrel{O H}{\rightleftharpoons}\left[C u\left(N H_{3}\right)_{4}\right]^{2+}\)

\(C u^{2+}(a q)+4 N H_{3} \stackrel{H}{\rightleftharpoons} C u^{2+}+4 N H_{4}^{+}\)