Final answer:
In the compound SO₃, the sulfur atom has an oxidation number of +4 to balance the -2 oxidation numbers from each of the three oxygen atoms, making option b. SO₃ the correct answer.
Step-by-step explanation:
The question is about determining in which compound an element has an oxidation number of +4. To assign oxidation numbers, certain rules are followed that help in apportioning electrons among the atoms in a compound.
For the compound SO₃, sulfur must have an oxidation number of +4 to ensure the sum of the oxidation numbers equals the charge on the species (which is zero in this neutral molecule). Each oxygen atom has a known oxidation number of -2 as per the rules. Therefore, since there are three oxygen atoms contributing a total of -6, sulfur must have an oxidation number of +4 to counteract this and maintain overall electrical neutrality.
Thus, considering the options provided, option b. SO₃ has sulfur with an oxidation number of +4. It's important to note that this doesn't imply that sulfur physically carries a +4 charge; it is simply a formal way by the concept of oxidation numbers to represent electron distribution in the molecule.