Answer: The atom which gains electrons is sulfur
Step-by-step explanation:
Oxidation reaction is defined as the chemical reaction in which an atom looses its electrons. The oxidation number of the atom gets increased during this reaction.

Reduction reaction is defined as the chemical reaction in which an atom gains electrons. The oxidation number of the atom gets reduced during this reaction.

For the given chemical equation:

Oxidation half reaction:

Reduction half reaction:

Hence, the atom which gains electrons is sulfur