Answer: Conduction
Explanation: MODES OF HEAT TRANSFER
Conduction: This type of heat transfer happens when there is direct contact between the two object.
Convection: This type of heat transfer happens when there is a movement of fluid (liquid or gas).
Radiation: This type of heat transfer happens when there is direct transfer of energy through space.
Thus as there is direct contact between the mug and the tea, the heat transfer occurs through conduction.