At standard pressure, during which physical change does the potential energy decrease?

Answer is: liquid to solid.

Cooling is change from liquids to solids.

A pure substance in the gaseous state contains more energy than in the liquid state, which in turn contains more energy than in the solid state.

Potential energy is type of stored energy.

A physical change occurs when a substance changes phase.

Physical property can be observed and measured without any changes in molecular composition. The same substance is present before and after the change.