Which molecule of a liquid has higher potential energy?
A) One at the center of gravity of the liquid
B) One at maximum distance from centre of gravity of liquid
C) One on the surface film
D) One at the bottom of the vessel

Hint: When a liquid is put in a container, each particle of the liquid is at a different position. Due to this variation in position, each particle exerts/experiences a different force from the other particle. Thus, every particle in the liquid has a different physical condition and thus, a different potential energy associated to it.

Complete answer:
Potential energy is the energy that is put away, or preserved, in an article or substance. This energy depends on the position, course of action or condition of the article or substance. You can consider it as energy that has the 'potential' to accomplish work. When the position, course of action or condition of the item changes, the potential energy also changes.

Attractive powers between atoms of a similar kind are called cohesive forces. Fluids can, for instance, be held in open compartments in light of the fact that durable forces hold the atoms together. Attractive forces between atoms of various sorts are called adhesive forces. Such powers make fluid drops to stick to window sheets, for instance. In this part we analyze impacts legitimately inferable from cohesive and adhesive powers in fluids.

Now all the molecules of a liquid stored in a container exert a force on other molecules. If a molecule is surrounded by the same kind of molecule, the force it exerts on those molecules, in all directions, is the same amount of force that it experiences by the same molecules on which it has exerted its force. Hence, net force becomes zero and hence, these molecules are stable. On the other hand, the molecules that are on the top of the liquid only experience force from below them, as no molecules of liquid are present above them. The net force here is not zero instead the net force is upward as there are no molecules of liquid above the molecules on the top to neutralize the force exerted.

Hence, these molecules become unstable which results in them gaining potential energy. Hence, they have the maximum potential energy as the molecules below them are stable and have zero potential energy.

Therefore, Option (C) is correct.

Note: This upward force exerted by the surface molecules is responsible for surface tension in liquids. This force is the reason why we see a little bulge on the surface film of liquids when kept in an open container.