Final answer:
Mechanical waves need molecules to transfer energy, while electromagnetic waves do not require a medium and can travel through a vacuum.
Step-by-step explanation:
The wave that needs molecules in order to transfer energy is a mechanical wave. Unlike electromagnetic waves, which can travel through a vacuum because they consist of fluctuating electric and magnetic fields, mechanical waves require a medium (such as air, water, or a solid) to propagate. Mechanical waves transfer energy through the vibration or movement of the molecules in the medium. Electromagnetic waves like light and microwaves, on the other hand, do not require a medium and can transfer energy through the vacuum of space, as they are made up of photons that carry energy corresponding to their frequency.