Which one of the following primarily affects natural convection?

Natural convection is primarily driven by buoyancy forces, which are the result of density differences within a fluid. When a fluid is heated, it becomes less dense and tends to rise, while cooler, denser fluid sinks. This movement creates a convection current.

In environments where heat is transferred through a fluid, such as air or water, the effect of buoyancy is crucial. As warmer fluid rises, it displaces cooler fluid, allowing for the continuous circulation of the fluid and the transfer of heat. This process is a fundamental aspect of natural convection, where the motion arises solely from temperature differences, rather than external influences or mechanical means.

The other options do not directly drive the natural convection process. External electric fields can have effects on charged particles or certain fluids but are not a primary factor in natural convection. Constant volume does not influence the buoyancy effect, and while pressure gradients can influence fluid motion, they are more pertinent to forced convection scenarios where external forces drive fluid flow. Therefore, buoyancy forces are the key element affecting natural convection.