The correct answer is B. force of cohesion.
Cohesion is the force that holds molecules of the same substance together. It is a type of intermolecular force that is responsible for the surface tension of liquids. Surface tension is the tendency of a liquid surface to contract into the smallest possible area. This is because the molecules at the surface of a liquid are attracted to the molecules below them, but not to the air molecules above them. This creates a net inward force that pulls the surface of the liquid together.
Adhesion is the force that holds molecules of different substances together. It is also a type of intermolecular force. Adhesion is responsible for the ability of water to stick to surfaces, such as glass.
Friction is the force that opposes the relative motion of two surfaces that are in contact. It is caused by the roughness of the surfaces and the adhesion between the molecules of the two surfaces.
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. It is a passive process that does not require energy.
In conclusion, the free surface of a liquid behaves like a sheet and tends to contract to the smallest possible area due to the force of cohesion.