The phenomenon of a puddle of water disappearing after a few days is due to evaporation. So the answer is A.
Evaporation is the process of water turning from a liquid to a gas. This happens when the water molecules at the surface of the puddle gain enough energy to escape into the atmosphere. The sun provides the energy for evaporation, so the puddle will evaporate faster on a hot day.
The other choices are incorrect. Option B is incorrect because ice is not transparent. Option C is incorrect because water gains energy from sunlight, not loses energy. Option D is incorrect because water freezes to ice when it loses energy, not gains energy. Option E is incorrect because water molecules do not shrink when they evaporate.