Explanation:

The reactivity of metals generally increases down a group and decreases across a period in the periodic table. All the given options are alkali metals (Sodium, Potassium) or alkaline earth metals (Calcium, Magnesium). Potassium ($\text{K}$) is in Group 1, Period 4, while Sodium ($\text{Na}$) is in Group 1, Period 3. Calcium ($\text{Ca}$) is in Group 2, Period 4, and Magnesium ($\text{Mg}$) is in Group 2, Period 3. Since reactivity increases down a group, Potassium is more reactive than Sodium. Also, Group 1 metals are generally more reactive than Group 2 metals due to having only one valence electron to lose. Therefore, Potassium is the most reactive among the given options. Option (C) is correct. Option (A) is incorrect; Sodium is reactive but less so than Potassium. Option (B) is incorrect; Calcium is an alkaline earth metal and less reactive than alkali metals like Potassium. Option (D) is incorrect; Magnesium is also an alkaline earth metal and less reactive than Potassium.