d
\(\overset{\,\,\centerdot \,\,\centerdot \,\,\,\,\,\,\,\,}{\mathop{\text{P}{{\text{H}}_{3}}}}\,+{{\text{H}}^{+}}\to \text{PH}_{4}^{+}\)

According to Drago's rule lone pair on phosphorus resides in almost pure s-orbital, hence due to non-directionalnature, its overlapping tendency is greatly reduced in comparison to a lone pair present in hybrid orbital, which is directional as present in \(\overset{\,\,\centerdot \,\,\centerdot \,\,\,\,\,\,\,\,}{\mathop{\text{N}{{\text{H}}_{3}}}}\,\).