The type of heat transfer that occurs only in liquids and gases is convection.

Here's why:

* Convection is the transfer of heat through the movement of fluids (liquids or gases). This movement is driven by differences in density. Warmer fluids are less dense and rise, while cooler fluids are denser and sink. This creates a circular flow that transfers heat.

* Conduction (transfer of heat through direct contact) and radiation (transfer of heat through electromagnetic waves) can occur in solids, liquids, and gases.

So, while conduction and radiation can occur in liquids and gases, convection is the unique heat transfer mechanism that only applies to fluids.