Answer
+4
Work Step by Step
H2SO3. Hydrogen is +1 (except in metal hydrides, which this is not) and we have 2 of them so that is +2. Oxygen is -2 so we have 3*-2 = -6.
Since H2SO3 is neutral, it means that the sum of the oxydation numbers is 0
Therefore, we have:
0 = 2 - 6 + Oxidation number Sulfur
Oxidation Number Sulfur = 6 - 2 = +4