Viscosity is a property of fluids that measures their resistance to flow. It is influenced by several factors, including surface tension, cohesion, intermolecular attraction, and vapor pressure.

I. Surface Tension:
Surface tension is the force acting on the surface of a liquid that tends to minimize its surface area. It is caused by the cohesive forces between the liquid molecules at the surface. Surface tension contributes to the viscosity of a fluid by affecting the flow of molecules at the surface. It creates a resistance to the movement of molecules, making the fluid more viscous.

II. Cohesion:
Cohesion refers to the intermolecular forces that hold the molecules of a substance together. In the case of a liquid, cohesive forces between the molecules contribute to its viscosity. These forces cause the molecules to resist flow and move past each other, resulting in a higher viscosity.

III. Intermolecular Attraction:
Intermolecular attraction refers to the forces of attraction between molecules. In a liquid, these forces play a significant role in determining the viscosity. Stronger intermolecular forces lead to a higher viscosity as the molecules are more strongly attracted to each other, resulting in a greater resistance to flow.

IV. Vapor Pressure:
Vapor pressure is the pressure exerted by the vapor of a substance in equilibrium with its liquid phase at a given temperature. While vapor pressure does not directly determine viscosity, it can indirectly influence it. An increase in vapor pressure can lead to a decrease in viscosity. This is because high vapor pressure tends to break the intermolecular forces and reduce the resistance to flow, resulting in lower viscosity.

Based on the above explanations, we can conclude that viscosity is primarily due to surface tension and cohesion (options a). Intermolecular attraction also contributes to viscosity, but vapor pressure does not have a direct impact on it. Therefore, the correct answer is option 'A' (II, III).