S. Being an oxidizing agent means that it can easily accept electrons. Sulfur is more electronegative than nitrogen. The periodic table contains the electronegativity data of elements. The other way to look at is is through their electron configuration. Sulfur is much more capable of accepting electrons, being deprived of just two more electrons for a complete octet. Nitrogen on the other hand needs 3 more, greater than that of sulfur. just get back here if you need more explanation.
