The correct answer to the question is B). Any medium i.e X can be any medium.
Before going to answer this question, first we have to understand various types of waves. Generally there are three types of waves which are known as mechanical wave, electromagnetic wave and matter wave.
A mechanical wave is a wave which requires a medium for its propagation. It may be a longitudinal wave or a transverse wave (except electromagnetic wave though it is transverse in nature).
The electromagnetic wave is a transverse wave . It has time varying electric field and magnetic field which are perpendicular to each other as well as perpendicular to the direction of propagation. It needs no material medium for its propagation. It can travel in space also.
Matter wave is the wave associated with any material particle in motion.It is also called as de- Broglie wave. It can travel in any medium.
As per the question, the wave is travelling through X. As the specific wave is not mentioned, so X can be any medium.