Which metal quickly reacts with cold water? 

Calcium 

The correct answer is option 4. Calcium.

Calcium is an alkaline earth metal that reacts vigorously with cold water. When calcium is added to cold water, it undergoes a single displacement reaction in which calcium displaces hydrogen from water, forming calcium hydroxide (\(Ca(OH)_2\)) and hydrogen gas (\(H_2\)).

The chemical equation for the reaction between calcium and cold water can be represented as follows:

\[ \text{Ca(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)} \]

In this reaction, calcium (\(Ca\)) reacts with water (\(H_2O\)) to produce calcium hydroxide (\(Ca(OH)_2\)) and hydrogen gas (\(H_2\)).

The reaction between calcium and water is highly exothermic, meaning that it releases a large amount of heat energy. This is why the reaction appears vigorous, with the formation of bubbles of hydrogen gas and the evolution of heat. The calcium hydroxide produced is sparingly soluble in water and forms a cloudy white precipitate, which can be observed in the reaction mixture.

This reaction is used in various industrial applications, such as in the production of calcium hydroxide for water treatment, in the preparation of hydrogen gas, and in certain types of chemical synthesis.