C. No. An element's reactivity decreases as atomic mass increases.
The correct answer is not C because an element's reactivity is actually determined by its position on the periodic table and its electron configuration, not just its atomic mass. The order in which the elements were arranged is also incorrect based on their positions on the periodic table. Helium (He) is the least reactive element and should be placed first, followed by carbon (C), magnesium (Mg), and then potassium (K), which is the most reactive among the four.