Explanation:

The phenomenon where a liquid surface behaves like a stretched elastic membrane, tending to minimize its surface area, is called surface tension. This force arises from the cohesive forces between liquid molecules. Option A, surface tension, is precisely why liquid drops adopt a spherical shape, which has the smallest surface area for a given volume. Option B, vapour pressure, is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases in a closed system, and it does not directly cause surface contraction. Option C, density, is a measure of mass per unit volume and is unrelated to the minimization of surface area. Option D, viscosity, is a measure of a fluid's resistance to flow, which also does not explain the tendency of a drop to contract.