Reactivity of Metals:
Metal reactivity refers to how easily a metal will react with other substances. The most reactive metals tend to lose electrons easily, while less reactive metals are less likely to undergo a chemical change.

Potassium (K):
Potassium is the most reactive metal among the options given. It is a Group 1 alkali metal, which means it has one electron in its outer shell. This electron is easily lost, making potassium highly reactive. When exposed to air or water, potassium quickly reacts to form potassium hydroxide and hydrogen gas. Due to its high reactivity, potassium is stored under oil to prevent it from reacting with moisture in the air.

Sodium (Na):
Sodium is also a Group 1 alkali metal like potassium and is known for its reactivity. It reacts vigorously with water to form sodium hydroxide and hydrogen gas. However, sodium is slightly less reactive than potassium.

Calcium (Ca):
Calcium is an alkaline earth metal found in Group 2 of the periodic table. It is less reactive than both potassium and sodium. Calcium reacts with water to form calcium hydroxide and hydrogen gas, but the reaction is not as vigorous as that of potassium or sodium.

Iron (Fe):
Iron is a transition metal and is much less reactive than the alkali metals like potassium and sodium. Iron does react with oxygen to form iron oxide (rust), but it requires high temperatures or the presence of catalysts for the reaction to occur at a noticeable rate.
In conclusion, potassium is the most reactive metal among the options provided. Its high reactivity is due to its position in Group 1 of the periodic table, which makes it easy for potassium to lose its outer electron and form compounds with other elements.