The material through which a mechanical wave travels is a. a medium. b. empty space. c. ether. d. air

Answer: Medium

The medium is the main factor that differentiates a mechanical wave from an electromagnetic wave, since the first can not propagate without its existence, while the second can propagate regardless of whether the medium exists or not.

In addition, it is the medium that will define, the propagation speed of the wave, according to its specific physical characteristics.

Therefore, the correct answer is a.