The correct answer is option 3: Magnesium (Mg).

Explanation: When a metal reacts with water, it can either displace hydrogen or produce a metal hydroxide and hydrogen gas. Calcium (Ca), Potassium (K), and Sodium (Na) are all highly reactive metals that react vigorously with cold water. These metals displace the hydrogen in water and form metal hydroxides along with hydrogen gas.

However, Magnesium (Mg) is a moderately reactive metal. It does react with water, but it does so very slowly. The reaction of magnesium with water is not as vigorous as that of calcium, potassium, or sodium. Instead of displacing hydrogen, magnesium reacts with water to form magnesium hydroxide and hydrogen gas. This reaction is much slower and less noticeable than the reactions of the other metals in cold water.

In summary, Magnesium (Mg) is the metal that does not react as vigorously with cold water compared to calcium, potassium, and sodium.