The correct answer is A. Conduction.
Conduction is the transfer of heat between two objects that are in direct contact with each other. The 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. When a fluid is heated, it expands and becomes less dense. This causes the hot fluid to rise, while the cooler fluid sinks. This movement of the fluid transfers heat from the hotter part of the system to the colder part.
Radiation is the transfer of heat through electromagnetic waves. Electromagnetic waves are a form of energy that can travel through space without a medium. When an object is heated, it emits electromagnetic waves. These waves carry heat energy to other objects, which can then absorb the heat energy.
Ultrasonic is a type of sound wave with 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 conclusion, the transfer of heat by molecule-to-molecule contact is conduction.