If an atom has lone pairs and bonded electron pairs, the greater lone- pair lone- pair electron repulsions will cause bond angle to decrease between bonded atoms.
NH−2 has 2 bond pairs and 2 lone pairs of electrons.
NH3 has 3 bond pairs and 1 lone pair.
NH+4 has 4 bond pairs and no lone pairs.
Bond- pair bond- pair electron repulsions < bond- pair lone- pair electron repulsions < lone- pair lone- pair electron repulsions.
Greater lone- pair lone- pair repulsions among electrons causes the H−N−H bond angle to decrease.
This effect is greatest for NH−2, followed by NH3.
Hence, the correct order in terms of HNH bond angle is as follows :
NH−2<NH3<NH+4
Therefore, the correct option is D.