Explanation

(b)
In $NH_{3}$, the N atom contains a one lone pair of electrons and three bond pairs in its valence shell. So, it shows $sp^{3}$ hybridisation. Due to presence of one lone pair of electron, its shape deviates from tetrahedral because lone pair shows more repulsion than bond pairs.
As
$lp\ –\ lp\ > \ lp\ - \ bp\ > \ bp\ - \ bp$
So, its shape is pyramidal and angle $107{^\circ}$.