Natural convection is a method of heat transfer where the movement of the fluids results from:

Natural convection occurs due to the movement of fluids, which is primarily driven by differences in density that arise from temperature variations within the fluid. When a fluid is heated, it typically becomes less dense and rises, while cooler, denser fluid descends to take its place. This continuous cycle creates a convection current that facilitates heat transfer through the fluid.

In this context, the phenomenon directly relates to the principles of buoyancy: as warmer fluid rises and cooler fluid sinks, it allows for the distribution of thermal energy. The efficiency of natural convection is significantly influenced by the temperature gradient within the fluid.

While gravitational differences and surface pressure can play roles in fluid movement, they do not fundamentally drive the process of natural convection as the difference in densities does. Similarly, the time taken for heating is not a primary factor in the driving force behind natural convection. The focus on density differences effectively captures the essence of why fluids move in response to heating, making it the correct choice.