The correct answer is: A. surface tension.
Surface tension is the tendency of a liquid surface to shrink into the smallest possible area. This is due to the attraction between the molecules of the liquid. The molecules at the surface of the liquid are not surrounded by other liquid molecules on all sides, so they experience a net inward force. This inward force causes the surface of the liquid to contract.
Viscosity is the resistance of a fluid to flow. It is caused by the friction between the molecules of the fluid. The higher the viscosity, the more resistance there is to flow.
Friction is the force that opposes the relative motion of two surfaces in contact. It is caused by the roughness of the surfaces and the adhesion between the surfaces.
Cohesion is the force that holds together the molecules of a substance. It is caused by the attraction between the molecules.
In conclusion, the free surface of a liquid tends to contract to the smallest possible area due to the force of surface tension.