<u>Answer:</u> The correct answer is fluids.
<u>Explanation:</u>
There are 3 modes of heat transfer that happens:
1. <u>Conduction:</u> In this type of heat transfer, heat is transferred when there is direct contact between the two object.
2. <u>Convection:</u> In this type of heat transfer, heat is transferred when there is a movement of fluid (liquid or gas).
3. <u>Radiation:</u> In this type of heat transfer, heat is transferred when there is direct transfer of energy through space.
Hence, convection is only restricted to fluids which are liquids and gases.