Which of the following options is not considered a form of heat transfer?

Heat transfer occurs through three primary mechanisms: convection, conduction, and radiation.

Convection involves the movement of fluids (liquids and gases) where warmer portions of the fluid rise while cooler portions sink, creating a circulation pattern that transfers heat.

Conduction is the process where heat is transferred through materials without any movement of the material itself, primarily occurring in solids where heat moves from one molecule to another.

Radiation is the transfer of energy through electromagnetic waves, allowing heat to move through a vacuum, such as the heat from the sun reaching the Earth.

Expansion, however, is not a method of heat transfer. Instead, it refers to the increase in volume of a substance when it is heated. While expansion can be a consequence of heat transfer (for example, when a substance absorbs heat and expands), it does not itself serve as a means through which heat is transferred from one object to another. Therefore, it is correct to identify expansion as not being a form of heat transfer.