Hydrolysis constant of $NH^+_4$ is $5.55 \times 10^{-10}$. The ionisation constant of $NH^+_4$ is

Question

(a) $1.8 \times 10^9$

(b) $5.55 \times 10^{-10}$

(c) $5.55 \times 10^4$

(d) $1.8 \times 10^{-5}$