Surface tension is due to cohesion.

Surface tension is a phenomenon that occurs at the interface between a liquid and a gas or another liquid. It is the force that acts on the surface of a liquid, causing it to behave like a stretched elastic sheet. The cohesive forces between the liquid molecules are responsible for surface tension.

Cohesion:
- Cohesion is the intermolecular force of attraction between like molecules. In liquids, these forces are responsible for holding the molecules together and giving the liquid its unique properties.
- In the case of surface tension, cohesion refers to the attraction between liquid molecules at the surface of a liquid.
- The cohesive forces between the liquid molecules pull the molecules at the surface inward, creating a net force that acts parallel to the surface.
- This net inward force causes the surface of the liquid to behave like a stretched elastic sheet, resulting in the phenomenon of surface tension.

Surface Tension:
- Surface tension is the property of a liquid that allows it to resist external forces and minimize its surface area.
- It can be thought of as the elastic tendency of the liquid's surface to contract and behave like a stretched membrane.
- Surface tension is responsible for various phenomena, such as the formation of droplets, capillary action, and the floating of certain objects on the surface of water.

Explanation of the options:
a) Viscous forces: Viscosity refers to the resistance of a fluid to flow. While viscosity is related to the internal friction between layers of a liquid, it is not directly responsible for surface tension. Therefore, option A is incorrect.
b) Cohesion: As explained above, cohesion is the intermolecular force of attraction between like molecules. These cohesive forces give rise to surface tension. Therefore, option B is the correct answer.
c) Adhesion: Adhesion refers to the attraction between different substances. While adhesion can play a role in certain surface tension phenomena, such as capillary action, it is not the primary cause of surface tension. Therefore, option C is incorrect.
d) The difference between adhesive and cohesive forces: While the difference between adhesive and cohesive forces can affect certain surface tension phenomena, such as the rise or fall of liquid in a capillary tube, it is not the fundamental cause of surface tension itself. Therefore, option D is incorrect.

In conclusion, surface tension is primarily due to the cohesive forces between liquid molecules at the surface, which create a net inward force and cause the liquid surface to behave like a stretched elastic sheet.