Answer: The correct answer is Option a.
Explanation: There are 3 modes of heat transfer:
1. Conduction: In this type, heat is transferred when there is direct contact between the two objects.
2. Radiation: In this type, heat is transferred when there is direct transfer of energy through space.
3. Convection: In this type, heat is transferred whenever there is a movement of fluid which is liquid or gas.
Absorption is not a mode of heat transfer.
Hence, heat which passes through liquids occurs by the process of convection. Therefore, the correct option is option a.