Which pair of fields comprises electromagnetic radiation?

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Electromagnetic radiation consists of electric and magnetic fields that vary over time and space and travel together as a wave. These two fields are perpendicular to each other and to the direction of propagation, and they sustain each other: a changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. Because they are interdependent in this way, radiation cannot be described by just one field component—the energy and the wave nature come from both parts evolving in step. The other options mix concepts that don’t form a radiation pair, such as gravity with electric, magnetic with thermal, or electric with chemical fields. In free space, the oscillating E and B fields carry energy and momentum and move at the speed of light, which is the hallmark of electromagnetic radiation.