Which one of the following is NOT an electromagnetic wave?

Waves are primarily classified into mechanical and electromagnetic waves based on their propagation requirements. Sound waves are mechanical waves that require a medium (solid, liquid, or gas) to travel, propagating through the compression and rarefaction of that medium [1]. In contrast, electromagnetic waves do not require a medium and can travel through a vacuum. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Light waves are specifically identified as transverse electromagnetic waves [1]. Radio waves and microwaves are also integral parts of this spectrum, characterized by their specific wavelengths and frequencies [2]. Therefore, while light, radio, and microwaves are electromagnetic in nature, sound is a mechanical wave and does not belong to the electromagnetic spectrum [2].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 5: Earths Magnetic Field (Geomagnetic Field) > Why Do Sound Waves Travel Faster In A Denser Medium Whereas Light Travels Slower? > p. 64
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Why Are Microwaves And Some Radio Waves Not Reflected By The Ionosphere? > p. 278