Which of the following is a requirement for sound to occur?

For sound to occur, a medium is essential because sound is a mechanical wave that propagates through vibrations in molecules. This medium can be a solid, liquid, or gas, and it serves as the substance through which sound waves travel. In the absence of a medium, sound cannot be generated or transmitted, as there would be no particles to carry the vibrations.

Other options, such as a heater, transmitter, and amplifier, may play roles in specific contexts related to sound technology or modification, but they are not fundamental requirements for the existence or propagation of sound itself. The foundational aspect of sound is its dependence on a medium to facilitate the movement of sound waves.