$SO_2+H_2O\rightarrow H_2SO_3$. $\quad H_2SO_3$ is also known as

Question

$\begin{array}{1 1}(a)\;Sulphuric\;acid\\(b)\;Sulphurous\;acid\\(c)\;Hydro\;sulphuric\;acid\\(d)\;Hypo\;sulphuric\;acid\end{array}$