Raindrops are spherical because of
A. Gravitational force
B. Surface tension
C. Air tension
D. Low viscosity of water

Hint: The spherical structure of the rain droplets is due to surface tension of water. The surface tension is like the skin of a body of water which makes the water molecules stick together. The spherical shape has the least possible surface area due to which it can resist any of the external forces in the atmosphere.

Complete step by step answer:
As the raindrop falls, the round shape gets lost. The raindrop becomes like the top half of a hamburger bun. This is due to the speed falling through the atmosphere.
At bottom of the water drop, the airflow is higher than the airflow at the top. A small air circulation disturbance creates less air pressure at the top. The raindrop remains spherical at the top due to the surface tension while the bottom gets more flattened out.

Raindrop typically ranges $100$ to $1000\,{m^3}$; Droplets usually are more numerous during drizzling. Usually, raindrops have a diameter larger than $4\,mm$, because they increase in size when they get to break up. The concentration decreases when the diameter of the droplets increases. Meteorologists classify rain according to the rate of rainfall. The moderate, hourly rates relating to light, and heavy rain are, respectively, less than $2.5\,mm$, $2.8\,mm$ to $7.6\,mm$, and more than $7.6\,mm$.
The coalescence of small water droplets of the rainfall that collide or from the melting of snowflakes and other ice particles as it falls into warm air near the ground.

Hence, the option (B) is correct.

Note:
The tendency of the liquid surface to shrink into the minimum possible surface area is known as surface tension. Surface tension allows the small insects denser than water to float or slide on water.