Question

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.

Answer 1

You can't decide from that statement alone — it depends on what kind of wave it is.

- Electromagnetic waves (light, radio, X‑rays, etc.) can travel through empty space and through materials, so X could be empty space or a material.
- Mechanical waves (sound, waves on a string, seismic waves) must have a material medium (solid, liquid or gas), so X cannot be empty space in that case.
- Further: transverse mechanical waves require a medium that supports shear (typically solids or surface waves on liquids); longitudinal mechanical waves (sound) can travel in gases, liquids or solids.

So ask which type of wave is meant. If the question means a mechanical wave, pick "any medium"; if it means an electromagnetic wave, pick "empty space" (or "any medium" as well).