$P^H$ of $0.1\;M$ Solution of weak acid is 3. The value of ionisation constant $Ka$ of acid is

Question

$\begin{array}{1 1} (a)\;3 \times 10^1 \\(b)\;1 \times 10^{-3} \\ (c)\;1 \times 10^{-5} \\ (d)\;1 \times 10^{-7} \end{array} $