The correct answer is option 'D', which states that only cohesive forces contribute to the reason behind the origin of surface tension.

Surface tension is defined as the cohesive forces between molecules at the surface of a liquid. These forces arise due to the attractive forces between the molecules of the liquid. The cohesive forces are primarily responsible for the origin of surface tension, while adhesive forces play a secondary role.

Cohesive forces:
- Cohesive forces refer to the attractive forces between molecules of the same substance. These forces are responsible for holding the molecules together and giving the liquid its internal strength.
- In a liquid, the molecules are in constant motion, and each molecule experiences attractive forces from neighboring molecules. These forces act in all directions, resulting in a net force of zero within the liquid.
- However, at the surface of the liquid, the molecules experience an imbalance of forces. The molecules within the liquid exert a net inward force on the molecules at the surface, resulting in a contractive force that tries to minimize the surface area.
- This cohesive force at the surface is what gives rise to surface tension. It is responsible for the formation of droplets, the ability of insects to walk on water, and other phenomena related to surface tension.

Adhesive forces:
- Adhesive forces refer to the attractive forces between molecules of different substances. These forces occur at the interface between two substances, such as a liquid and a solid or two immiscible liquids.
- Adhesive forces can contribute to surface tension to some extent, but they are not the primary reason behind its origin. Adhesive forces can modify the behavior of surface tension, such as when a liquid wets a solid surface or when two immiscible liquids form droplets.
- However, in the absence of any adhesive forces, surface tension can still exist due to cohesive forces alone. Adhesive forces are not necessary for the existence of surface tension.

In conclusion, while both cohesive forces and adhesive forces can affect the behavior of surface tension, the primary reason behind the origin of surface tension is the cohesive forces between molecules within the liquid. Hence, option 'D' is the correct answer.