Final answer:
Sulfur (S) in the 3rd period is the element most likely to form an ion with a -2 charge, as it is located in Group 16 of the periodic table.
Step-by-step explanation:
The element in the 3rd period that is most likely to form an ion with a -2 charge is sulfur (S). According to the periodic table trends, main-group elements to the right of the periodic table tend to form anions with a negative charge that correlates with how many groups they are away from the noble gases.
Sulfur is in Group 16, which is two groups away from the noble gases, indicating it typically forms a 2- ion. In contrast, calcium (Ca) and magnesium (Mg) are Group 2 elements and will form 2+ cations, while selenium (Se) is in the same group as sulfur and will also form a 2- ion. However, among the choices given, sulfur is the one that more predictably forms a 2- charged ion in binary ionic compounds.