The correct option is C Magnesium
Here the elements are from two groups (group 1 and group 2). Among the given elements, sodium and potassium belong to group 1. The metallic characteristics increase down a group and since in group 1, sodium is the element that is present above potassium, it has lesser metallic character than potassium. Similarly in group 2, magnesium has lesser metallic character when compared to calcium. Now, sodium and magnesium belong to the same period and the metallic characteristics decrease along a period. Therefore, magnesium has the least metallic character among the given options.