Question

what do all electromagnetic waves have in common?
they can travel at the speed of light.
they have the same wavelengths.
they travel only through matter.
they have nonmoving magnetic fields.

Explanation:

Electromagnetic waves are a combination of electric and magnetic fields that oscillate. In a vacuum, all electromagnetic waves travel at the speed of light ($c = 3\times10^{8}\text{ m/s}$). Different electromagnetic waves (like radio waves, visible light, X - rays) have different wavelengths (from very long for radio waves to very short for X - rays). Electromagnetic waves do not need a medium (matter) to travel (they can travel in a vacuum). Also, the magnetic field in an electromagnetic wave is not non - moving; it is oscillating (changing sinusoidally) as the wave propagates. So the only property common to all electromagnetic waves among the given options is that they can travel at the speed of light.

Answer:

They can travel at the speed of light.