Final answer:
Potassium (K), being an alkali metal, is the most reactive element among those listed due to its propensity to lose its single valence electron and react vigorously, especially with water.
Step-by-step explanation:
Among the elements listed, Potassium (K) is the most reactive. This is because Potassium is part of the alkali metals group, which includes elements such as lithium (Li), sodium (Na), and cesium (Cs). Alkali metals are known for being extremely reactive, especially with water, forming basic (alkaline) solutions. They are so reactive due to their single valence electron which they can easily lose to form cations and engage in chemical reactions.
Comparatively, Zinc (Zn) and Iron (Fe) are less reactive, reacting slowly with air due to the formation of a passivating protective oxide coating. Calcium (Ca) is part of the alkaline earth metals group and is more reactive than zinc and iron but less reactive than potassium. Alkaline earth metals like calcium also react with water, though not as vigorously as the alkali metals.
Thus, based on the activity series and their position in the periodic table, the reactivity order from least to most reactive for the listed elements is Iron (Fe) < Zinc (Zn) < Calcium (Ca) < Potassium (K).