The falling drops of the rain acquires spherical shape because of:

Concept:

Surface tension:

Surface tension is due to cohesion between particles at the free surface.

Some examples of the phenomenon of surface tension are as follows:

• Raindrops become spherical (A falling raindrop becomes spherical due to cohesion and surface tension).
• Rise of sap in a tree.
• Birds can drink water from ponds.
• Capillary rise and capillary siphoning.
• Collection of dust particles on the water surface. 
• Break up of liquid jets.

​Hence option (2) is the correct answer.