The letter X in the table would be best replaced with the question "How are they formed?" This is because the formation of the waves is a crucial property that distinguishes how electromagnetic waves and mechanical waves are generated.
Thus, the completed table would look like this:
| Property | Electromagnetic | Mechanical | |----------------------|-------------------------------|------------------------------------| | Do they require a medium? | No | Yes | | How do they travel? | Transverse waves | Longitudinal and transverse waves | | How are they formed? | From varying electric and magnetic fields | From vibrating particles of the medium |