Final answer:
In H₂S, sulfur has the smallest oxidation number of -2.
Step-by-step explanation:
The oxidation number of sulfur is the same as its charge in a compound. In order to determine which molecule has sulfur with the smallest oxidation number, we need to look at the charge on sulfur in each molecule.
a) In H₂S, sulfur has an oxidation number of -2.
b) In SO₂, sulfur has an oxidation number of +4.
c) In SF₆, sulfur has an oxidation number of +6.
d) In S₈, sulfur has an oxidation number of 0.
Therefore, the molecule in which sulfur has the smallest oxidation number is H₂S (option a).