Which of the following is NOT part of the electromagnetic spectrum?

The electromagnetic spectrum includes all types of electromagnetic radiation, which travels in waves and includes various forms such as infrared, X-rays, and microwaves. Each type of radiation within the spectrum is characterized by its wavelength and frequency, and they all share the common property of being able to travel through a vacuum.

In contrast, sound waves are mechanical waves that require a medium (like air, water, or solids) to propagate. They cannot travel through a vacuum, which fundamentally distinguishes them from electromagnetic waves. Therefore, sound waves do not belong to the electromagnetic spectrum, making this answer correct. Understanding the distinction between these forms of waves is crucial in physics and helps clarify concepts related to energy, waves, and their interactions with matter.