Answer: The correct answer is "Must involve temperature differences between substances or objects".
Step-by-step explanation:
In conduction , there is no actual movement of matter. Heat flows from the hotter body to the colder body but when two bodies are placed in contact with each other.
In convection, there is a physical movement of matter.
Radiation is the emission of the energy. Heat travels through a vacuum in the radiation. There is no mass exchange occur. It does not rely upon the contact between hotter body and the colder body.
Therefore, the label belongs in the area marked Y is "Must involve temperature differences between substances or objects".