The correct answer is option 2, convection only. Convection is a mode of heat transfer that involves the movement of warmer material to displace cooler material. In convection, heat is transferred through the movement of fluids or gases. This can occur through natural convection, where the movement is driven by density differences caused by temperature variations, or through forced convection, which involves the use of mechanical devices such as fans or pumps to induce fluid movement.

Option 1, conduction only, is not the correct answer. Conduction refers to the transfer of heat through direct contact between materials or particles. It does not involve the movement of materials as in the given scenario.

Option 3, radiation, is also not the correct answer. Radiation is a mode of heat transfer that occurs through electromagnetic waves. It does not require a material medium for heat transfer and can occur in vacuum as well. However, radiation does not involve the bulk movement of materials as mentioned in the question.

Option 4, both conduction and convection, is not the correct answer. Though conduction can occur alongside convection in certain scenarios, the question specifically asks for the mode of heat transfer that involves the displacement of cooler material by warmer material, which is convection.