Identity a metal based on its properties
Copper is much denser than water and it does not react with it. Magnesium is also denser than water and reacts only very slowly. Calcium will also sink in water, although it will react with it producing hydrogen, so small pieces of the element may float on the surface. However, it would definitely not melt. The correct answer is potassium - it is a very soft metal, less dense than water and reacts with it producing enough heat to melt it (potassium melts at only $\pu{63.5^\circ C}$).