Out of the given options, calcium is the metal that floats in cold water. The reason for this is that calcium has a low density compared to water, causing it to float. Density is a measure of how much mass is packed into a given volume. If an object is less dense than the fluid it is placed in, it will float.

In this case, calcium has a lower density than cold water. When calcium is placed in water, the buoyant force acting on it is greater than its weight, causing it to float on the surface. This is due to the fact that calcium is a relatively light metal, with a density of 1.55 g/cm³, while the density of water is around 1 g/cm³.

On the other hand, metals like magnesium, potassium, and copper have higher densities than cold water, causing them to sink when placed in it. Magnesium and potassium have densities of around 1.74 g/cm³ and 0.86 g/cm³ respectively, while copper has a density of 8.96 g/cm³.

Therefore, the correct answer is option 2: Calcium.