The spherical shape of rain droplets is due to-

1. Viscosity

2. surface tension

3. Critical phenomenon

4. Pressure

Hint:  The tendency of liquid surfaces at rest to shrink into the minimum surface area possible. 

The spherical shape of rain droplets is due to surface tension. The lowest energy state of a liquid will be when the surface area is minimum. Surface tension tries to decrease the surface area of the liquid to the minimum. The rain droplets are spherical because for a given volume, a sphere has minimum surface area.