Final answer:
The bathtub full of cool water has more internal energy due to its larger mass and heat capacity.
Step-by-step explanation:
The bathtub full of cool water has more internal energy than the cup of hot water.
Internal energy is the total kinetic and potential energy of the particles within a substance. It depends on both the mass and temperature of the substance. Although the cup of hot water has a higher temperature, the bathtub full of cool water has a much larger mass. The larger mass of the bathtub full of cool water gives it a higher heat capacity, which means it can store more energy.
Therefore, even though the cup of hot water has a higher temperature, the bathtub full of cool water has more internal energy.