Option [a] is correct as--->
Presence of gravitational field is required for heat transfer by conduction.

The correct answer is option 'C', natural convection.

Natural convection:
Natural convection is a method of heat transfer that occurs due to the movement of fluid caused by temperature differences. It is driven by buoyancy forces resulting from density variations in the fluid. In natural convection, the fluid near a heat source becomes hotter and less dense, causing it to rise. As it rises, cooler and denser fluid takes its place, creating a continuous circulation of fluid.

Gravitational field and natural convection:
The presence of a gravitational field is required for natural convection to occur. This is because the buoyancy forces responsible for driving the fluid movement are directly related to the gravitational field.

Explanation:
1. Density variation: When a fluid is heated, it expands and becomes less dense. The lighter hot fluid rises while the cooler and denser fluid sinks. This density variation is the driving force behind natural convection.

2. Buoyancy force: The density difference between the hot and cold fluid generates a buoyancy force. This force acts in the opposite direction to gravity. In the presence of a gravitational field, the buoyancy force causes the hot fluid to rise and the cold fluid to sink.

3. Fluid circulation: As the hot fluid rises, it carries heat away from the heat source. This creates a circulation pattern known as a convection current. The cooler fluid replaces the hot fluid near the heat source, and the process continues, resulting in a continuous flow of fluid.

4. Gravitational field's role: The gravitational field provides a reference direction for the buoyancy forces. Without a gravitational field, there would be no clear upward or downward direction for the fluid movement. The fluid would not experience the buoyancy forces necessary for natural convection.

5. Heat transfer: Natural convection is an efficient method of heat transfer as it involves the bulk movement of fluid. The circulating fluid carries the heat away from the heat source, resulting in effective heat transfer.

In conclusion, the presence of a gravitational field is necessary for the occurrence of natural convection. Without a gravitational field, the buoyancy forces responsible for driving the fluid movement would not exist, and natural convection would not occur.