Sulfur could complete its octet by gaining two electrons. Sulfur is right under oxygen in group 16 on the periodic table of the elements. All the elements in this group need two electrons to complete the octet of electrons in their outer energy level of electrons. Group 16 is known for being electronegative and willingly accepting two electrons from electron donating elements, many of which are metallic in nature.
Of the other choices, sodium (Na) is from group 1. It donates it's one electron gladly to react with other elements. Calcium (Ca) is from group 2 and donates its two electrons to have its octet. Aluminum is in group 13, which is the Boron group, and donates its three electrons to bond with other elements. The last element, nitrogen (N), is in group 15, and is the first element in the nitrogen group, which accepts three electrons to form bonds with other elements and complete its octet.