Final answer:
In fluids, thermal energy is primarily transferred through convection, a process involving the movement of fluid due to temperature variations. Conduction occurs via direct contact, often in solids, while radiation transfers heat through electromagnetic waves.
Step-by-step explanation:
The type of thermal energy transfer that occurs mostly in fluids is convection. Convection is the heat transfer by the macroscopic movement of a fluid, such as air or water. This process includes the circulation of warm fluid rising and cool fluid sinking, which can often be seen in weather systems or heating systems like a forced-air furnace.
Conduction is another form of heat transfer and it involves the transfer of heat through direct contact, such as when a pan heats up on a stove because of its contact with the burner. It is most efficient in solids where molecules are closely packed. Lastly, radiation is the transfer of energy through electromagnetic waves, such as the heat felt from the Sun or a light bulb.