Explanation

(c)
(a) $\ \ \ \ \ \ \ \ CH_{4}\ \ \ \ \ \ \ \ \ \ \longrightarrow \ \ \ \ CH_{3} - \ CH_{3}$
4bp + 0lp 4bp 4bp
Hybridisation $sp^{3}$ $sp^{3}$ $sp^{3}$
Structure tetrahedral tetrahedral
(b)$\ \ \ \ \ \ \ \ NH_{3}\ \ \ \ \ \ \ \ \ \ \longrightarrow \ \ \ \ {NH}_{4}^{+}$
3bp + 1lp 4bp
Hybridisation $sp^{3}$ $sp^{3}$
Structure pyramidal tetrahedral
(c)$\ \ \ \ \ \ \ \ BF_{3}\ \ \ \ \ \ \ \ \ \ \ \longrightarrow \ \ \ \ {BF}_{4}^{-}$
3bp 4bp
Hybridisation $sp^{2}$ $sp^{3}$
Structure trigonal tetrahedral
planar
(d)$\ \ \ \ \ \ \ \ H_{2}O\ \ \ \ \ \ \ \ \ \ \longrightarrow \ \ \ \ H_{3}O^{+}$
2bp + 2lp 3bp + 1lp
Hybridisation $sp^{3}$ $sp^{3}$
Structure angular pyramidal
Thus conversion of $BF_{3}$ into ${BF}_{4}^{-}$ involves change in both hybridisation and shape.