Final answer:
Radiation travels in waves and does not require a medium to propagate. Electromagnetic waves, such as light from the Sun, can travel through a vacuum, unlike mechanical waves, which need a medium. The correct option is a).
Step-by-step explanation:
The student's question pertains to how radiation travels and what it requires for propagation. Radiation travels in waves.
Unlike convection and conduction, radiation does not need a medium to propagate. Therefore, the correct answer is a) Waves; Medium.
When considering different types of waves, not all require a medium. Specifically, electromagnetic waves do not require any medium to propagate. This includes light from the Sun, which reaches Earth through the vacuum of space. In contrast, mechanical waves, like sound waves, do require a medium to move through. Option a) is the correct one.