Answer: Ca, Mg, S, Cl
Explanation: general trends are IE increases as the number of electrons in the outermost shell, and decreases as the number of shells (and shielding effect) increases.
S outer 6, shells 3
Ca outer 2, shells 4
Mg outer 2, shells 3
Cl outer 7, shells 3
Ca has 2 outer, 4 shells —> lowest
others have 3shells
Mg 2outer, S 6, Cl 7
