(d)
$K_{h} = \frac{K_{w}}{K_{b}}$
where $K_{w} =$ionic product of water
$= 1 \times 10^{- 14}$
$K_{b} =$degree of dissociation of$\ NH_{4}OH = 1.8 \times 10^{- 5}$
$K_{h} = \frac{1 \times 10^{- 14}}{1.8 \times 10^{- 5}}$
$= 0.555 \times 10^{- 9}$
$= 5.55 \times 10^{- 10}$