Sulphurous acid turns: 

When sulphuric acid is dissolved in water, two hydrogen atoms are released, making ${\mathrm{H}}_2{\mathrm{SO}}_3$ an acid. Blue litmus becomes red when exposed to acid, while red litmus becomes blue when exposed to base.
${\mathrm{H}}_2{\mathrm{SO}}_3$, therefore, causing the color of blue litmus paper to change to red. Because sulphuorous acid releases two hydrogens when dissolved in water, it is a weak acid and is often referred to as dibasic acid. The most common reducing and disinfecting agents are sulfuric acid and its corresponding salts. In place of chlorine, sulfuric acid can be used to treat materials that are sensitive to chlorine as a light bleaching agent. Any acidic substance will turn blue litmus paper to red. It is a test for identifying the pH of substances. Thus sulphuorous acid will turn blue litmus paper to red.

Hence, option A is correct.