# In which there is change in the type of hybridisation when

$\begin{array}{1 1}(a)\;AlH_3\text{ combines with }H^-&(b)\;NH_3 \text{ combines with }H^+\\(c)\;\text{in both cases}&(d)\;\text{in none cases}\end{array}$

$NH_3$ combines with $H^+$ then $NH_4^+$($sp^3$) hybridisation)
Here there is no change in hybridisation.
$AlH_3$ combines with $H^-\rightarrow AlH_4^-$
$AlH_3\rightarrow(sp^2)$
$AlH_4^-\rightarrow(sp^3)$ hybridisation
Here there is a change in hybridisation.
Hence (a) is the correct answer.