The correct answer is A. Conduction.
Conduction is the transfer of heat between two objects that are in direct contact with each other. Heat is transferred from the hotter object to the colder object until the two objects reach the same temperature.
Convection is the transfer of heat through the movement of fluids. Hot air rises, while cold air sinks. This movement of air causes heat to be transferred from one place to another.
Radiation is the transfer of heat through electromagnetic waves. Heat can be transferred through radiation even if there is no direct contact between the two objects.
Ultrasonic is a type of sound wave that has a frequency above the range of human hearing. Ultrasonic waves can be used to transfer heat, but this is not a common method of heat transfer.
In the case of the question, the transfer of heat is happening by molecule-to-molecule contact. This is a type of conduction.