A string's vibration or loudspeaker cone's movement pushing air molecules forward demonstrates how sound waves are _____.

The correct concept to understand here relates to the generation of sound waves. When a string vibrates or when the cone of a loudspeaker moves, it disturbs the surrounding air molecules. This disturbance creates pressure changes in the air, which form sound waves. The action of the vibrating string or moving loudspeaker cone is responsible for the initial creation of those sound waves that later travel through the air.

Generation refers specifically to the process of creating sound, as the physical motion of the vibrating element causes the disturbances necessary for sound wave creation. While sound waves do indeed travel, transmit, and can be reflected, the focus of the question is on the origin of the sound waves and the activity that initiates their propagation through a medium. Therefore, generation is the most appropriate term to describe this phenomenon.