Correct Option (A):
The tendency of a liquid drop to contract and occupy the minimum possible surface area is a direct consequence of surface tension. Surface tension is a property of liquids that arises from the cohesive forces between molecules at the liquid-air interface. These forces create an inward pull on the surface molecules, causing the surface to behave like a stretched elastic membrane. To minimize its potential energy, the liquid surface attempts to reduce its area, leading to the contraction of drops into a spherical shape, as a sphere has the smallest surface area for a given volume.
Incorrect Options:
- Viscosity refers to the internal resistance of a fluid to flow. It governs the rate at which a liquid deforms under shear stress but does not directly cause a drop to contract its surface area.
- Density is defined as mass per unit volume. While it influences buoyancy and other gravitational effects, it does not explain the phenomenon of surface area minimization in a liquid drop.
- Vapour pressure is the pressure exerted by the vapour of a liquid in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. It is unrelated to the tendency of a liquid drop to contract its surface area.