There are three ways in which heat be transferred - conduction, convection, and radiation. Which types of thermal transfer involves fluids of different temperatures and densities?

Convection.

Explanation:

Let us define each of the type of thermal transfer first:

- radiation is heat transfer that does not include contact between the objects. It's mediated by infra-red radiation, waves from the invisible radiation spectrum.

- conduction is heat transfer which is mediated through direct contact between objects (holding a hot cup of tea, for example)

- convection is a heat transfer found in gases and liquids. Due to different temperatures in two parts of liquid, there will also be a different density (hotter parts have lower density). Lower density parts will start moving upwards while higher density parts, due to gravity, will move downward. As they move, they will gain or receive heat, which will cause new temperature differences and the moving will restart.