In heat transfer, which method is based on gravitation
$A)$ Natural convection
$B)$ Conduction
$C)$ Radiation
$D)$ Stirring of liquids

Hint: Stirring a liquid does not transfer heat, while conducting heat requires the source to be close to the sink. Natural convection and radiation do not need a medium to transfer heat into a system, and neither do they need the source to be close to the object.

Complete answer:
Conduction, convection, and radiation are the three methods for transferring heat in a system.
Stirring liquids is not a method of heat transfer in a system.
During conduction, when the source comes into contact with the object, heat is transferred directly to the object's molecules, which causes the molecules to be excited and rise, while the less heated molecules settle down and then become excited. No gravitational field is required for this type of transfer.
The heat transfer in convection does not require any contact. Thermal energy is transferred from the source to the object. Because the temperature gradient only exists in the vertical direction and not in the horizontal, particles move up and down depending on their weight and gravity.
As a result, in convection heat transfer, the presence of a gravitational field is required.
Therefore, option $A)$ Natural Convection is the correct answer.

Note:
Radiation travels without the use of a medium. When there is no medium between the earth and the sun, for example, we feel the warmth of the sun because heat travels through radiation. That is, there is only space between them. In a vacuum, conduction and convection cannot occur.