$\begin{array}{1 1} \text{(a) Between $NH_3$ and $PH_3$, $NH_3$ is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional} \\ \text{(b) Between $NH_3$ and $PH_3$, $PH_3$ is a better electron donor because the lone pair of electrons occupies spherical $sp^3$ orbital and is more directional} \\ \text{(c) Between $NH_3$ and $PH_3$, $NH_3$ is a better electron donor because the lone pair of electrons occupies spherical $sp^3$ orbital and is more directional}\\ \text{(d) Between $NH_3$ and $PH_3$, $PH_3$ is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional} \end{array}$
