Explanation:

- Let’s look at the reactivity of each metal:

- Copper: Least reactive among these. Hardly reacts with water or dilute acids.

- Calcium: Most reactive in this list! Reacts quickly with water and acids, producing hydrogen gas. It is found higher up in the reactivity series.

- Zinc: Reacts with dilute acids, but less vigorously than calcium.

- Iron: Less reactive than zinc and calcium; rusts slowly with moisture and air.

- So, the most reactive metal here is Calcium.