Natural convection is caused by which of the following?

Natural convection occurs due to differences in fluid density that arise from temperature variations within a fluid. When a fluid is heated, it becomes less dense and rises, while cooler, denser fluid descends. This process creates a circulation pattern known as convection currents. This phenomenon is vital in many heating and cooling systems, as it can enhance the transfer of heat without the aid of mechanical means like fans or pumps.

In contrast, the other options involve active mechanical processes or systems designed to induce airflow or heat exchange. The movement of air due to fans and forced air systems are examples of forced convection, where external forces drive the fluid movement. Heat exchangers facilitate heat transfer but do so through conduction and convection facilitated by the design rather than natural convection alone. Thus, the correct understanding of natural convection centers on the relation between fluid density and temperature changes.