C. Potassium
Explanation: Sodium Na and Potassium K are present in the group I that is alkali metal elements. While Magnesium Mg and Calcium Ca are group II alkaline earth metals. As we move from left to right in the periodic table the ionization energy increases as new electrons enter the same shell so, effective nuclear charge increases. So, Na and K are more reactive than Mg and Ca respectively. And as we go down the group number of shells increases so atomic size increases so, ionization energy decreases. As ionization energy decreases the reactivity of metal increases. So, Potassium is the most reactive metal.