In which of the following ways of heat transfer no medium is required?

Radiation

The correct answer is option 4. Radiation.

In only one of the ways of heat transfer is no medium required: Radiation

Radiation: Heat transfer through radiation happens via electromagnetic waves. These waves can travel through a vacuum, meaning they don't require any intervening material for propagation. This is how heat from the Sun reaches us across the vast emptiness of space.

Conduction: Conduction involves the transfer of heat through direct contact between objects. The microscopic particles of the objects vibrate and collide, transferring thermal energy from hotter to colder objects. This process requires a medium for the vibrations to travel through.

Convection: Convection is the transfer of heat through the movement of fluids (liquids or gases). As a fluid gets heated, it expands and becomes less dense, rising. Cooler, denser fluid then sinks to take its place. This creates a circular motion within the fluid, transferring heat from the hot region to the cold region. Convection also requires a medium (the fluid) to move heat.

Therefore, radiation is the only heat transfer method that doesn't necessitate a medium.