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

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

Sulphurous acid is the strong acid produced by dissolving sulphur dioxide in water.
$SO_2+H_2O\rightarrow H_2SO_3$
Sulphurous acid, $H_2SO_3$, is a weak dibasic acid, known in the form of its salts (e.g. sodium sulphite).
Hence (b) is the correct answer.
edited Mar 29, 2014