b
The hydrides have a pyramidal or tetrahedral shape with a lone pair of electrons in one of the orbitals. The \(H -M -H\) bond angle is less than the original \(109^o28'\) tetrahedral bond angle \((H -N -H\) in \(NH_3\) is \(106^o45')\) because of greater repulsion between lone pair and a bond pair than between two bond pairs of electrons. Because electro-negativity of \(M\) decreases from \(N\) to \(Bi\) , the bond pair lie farther away from the central atom, and the lone pair causes greater distortion of bond angle. Thus \(H -P -H\) bond in \(PH_3\) is \(94^o\) , while in \(AsH_3\) and \(SbH_3\) it is about \(91.8^o\) and \(91.3^o\) respectively (closer to \(90^o\) ). This suggests that orbitals used for bonding are closer to pure \(p-\) orbitals