Final answer:
Sound waves are caused by vibrations in a medium, which are detected by the ear and interpreted as sound by the brain.
Step-by-step explanation:
Sound is a type of energy that travels in waves, which are disturbances of matter transmitted from their source outward. The correct answer to what causes sound waves is C: vibrations. When an object vibrates, it causes the surrounding medium (such as air, water, or solids) to vibrate as well, and these vibrations travel as sound waves. For example, when a rock is thrown into a pond, it creates ripples in the water, and similarly, when a tuning fork is struck, it causes air molecules to oscillate, transmitting sound. The ears detect these sound waves by sensing the vibrations they cause, which are then converted into nerve impulses that the brain perceives as sound.