To determine which metal is the most reactive, we can reference the reactivity series of metals. The reactivity series ranks metals based on their ability to displace hydrogen from water or acids, and to displace other metals in single displacement reactions.

1. The reactivity series lists metals in the following order (from most reactive to least reactive): Potassium, Sodium, Calcium, Magnesium, Aluminium, (and so on). Metals like copper, iron, and zinc follow these more reactive metals. In the provided options, calcium is placed higher than copper, iron, and zinc in the reactivity series.
2. Copper is a low-reactivity metal and is placed near the bottom of the reactivity series, which is why it is used for electrical wiring and plumbing because it resists corrosion.
3. Iron is more reactive than copper but less than calcium or zinc.
4. Zinc is above copper and iron, making it more reactive than them but still less reactive than calcium.
5. Calcium, being higher up in the reactivity series than zinc, iron, and copper, is the most reactive metal among the given options.

Therefore, the correct answer is Calcium. This conclusion is based on the established order of metal reactivity, identifying calcium as the most prone to participation in chemical reactions, such as displacing other elements from compounds or reacting vigorously with water.