Which one of the following is not an electromagnetic wave?

Electromagnetic (EM) waves are transverse waves produced by oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel. They need no material medium and always propagate through vacuum at the speed of light. In increasing order of frequency, the electromagnetic spectrum runs: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

• Infrared lies within the electromagnetic spectrum, just below visible light in frequency.

• X-rays lie within the electromagnetic spectrum, beyond ultraviolet at the high-frequency end.

• Microwave lies within the electromagnetic spectrum, between radio waves and infrared.

• Ultrasonic is a mechanical (sound) wave with frequency above 20 kHz — above the range of human hearing. Unlike an electromagnetic wave, it needs a material medium (solid, liquid, or gas) to travel and cannot pass through vacuum.

The defining physical test confirms this: an electromagnetic wave can travel through vacuum (sunlight reaching Earth through space), while a mechanical/sound wave cannot (there is no sound in space). Applying this test, Ultrasonic is the only option that fails to qualify as an electromagnetic wave, so it is the correct answer.