Final answer:
Radiation is the method of heat transfer that does not require a medium and can occur through empty space. Earth is warmed by absorbing electromagnetic radiation emitted by the sun.
Step-by-step explanation:
Radiation is the only method of heat transfer where no medium is required, meaning that the heat doesn't need to come into direct contact with or be transported by any matter. The space between Earth and the sun is largely empty, without any possibility of heat transfer by convection or conduction. Instead, heat is transferred by radiation, and Earth is warmed as it absorbs electromagnetic radiation emitted by the sun.