The spherical shape of rain-drop is due to

The correct answer is Surface tension.

Let's understand why a rain-drop takes a spherical shape step-by-step:
1. Concept of Surface Tension: Surface tension is the property of a liquid surface that causes it to behave like a stretched elastic membrane. It arises due to cohesive forces between liquid molecules. Molecules inside the liquid are attracted equally in all directions by neighboring molecules, whereas molecules on the surface experience a net inward force pulling them toward the bulk of the liquid.
2. Minimizing Surface Area: Due to this inward pull, the liquid naturally tries to minimize its free surface area to achieve a state of minimum potential energy.
3. Geometric Advantage of a Sphere: For a given volume, a sphere has the smallest surface area compared to any other three-dimensional shape.
4. Result: Therefore, to minimize its surface area and surface energy, a falling rain-drop assumes a spherical shape.