The oxidation number of an atom is a measure of the number of electrons it has gained or lost, assuming that the electrons in a chemical bond are assigned to the more electronegative atom. In the compound H_{2}SO_{3}, there are two hydrogen atoms, each with an oxidation number of +1, and one oxygen atom, which has an oxidation number of -2. The sum of the oxidation numbers in H_{2}SO_{3} is zero, since the compound is neutral. Therefore, we can calculate the oxidation number of sulfur as follows: 2(+1) + x + 3(-2) = 0 Solving for x, we get: x = +4 Therefore, the oxidation number of sulfur in H_{2}SO_{3} is +4. In summary, the correct option in the question is H_{2}SO_{3}, where the oxidation number of sulfur is +4.