Question

a wave is traveling through x. what can be known about x? it is empty space. it is any medium. it is a solid. it is a liquid.

Explanation:

Waves can travel through solids, liquids, gases and other media. Empty space does not allow wave - travel for mechanical waves (though electromagnetic waves can travel through it). Since a wave is traveling through X, X must be some kind of medium.

Answer:

It is any medium.