Gravitational force is essentially required for

Convection is a method of heat transfer that generally takes place in fluids, for example, water.
When water is heated, hot water molecules near the burner go up upon heating and cold water molecules come down.
It is caused by gravity which pulls heavier elements or more dense colder elements near the surface of liquids or in gases downwards, causing lighter elements to go upwards.
Therefore, option B is the correct answer.