B. travels in a vacuum
Explanation:
Electromagnetic waves are periodic oscillations of an electric field and a magnetic field, which occur perpendicular to the direction of propagation of the wave. The main property of these waves is that they do not a medium to propagate: in fact, electromagnetic waves can also travel in a vacuum (e.g. they can travel in space).
On the contrary, mechanical waves are oscillations of the particles in a medium, so they do need a medium to propagate.
Therefore, the main difference between electromagnetic waves and mechanical waves is that the former can travel in a vacuum, while the latter cannot.
