What distinguishes diegetic sound from non-diegetic sound in film?

Diegetic sound refers to sounds that originate from the world of the film itself, meaning they are sounds that the characters can hear. This includes dialogue, sound effects from actions and environmental noises, all of which exist within the context of the film's narrative. Non-diegetic sound, on the other hand, pertains to all audio elements that do not have an origin within the film's world, such as background music, voiceovers, or soundtrack music that the characters cannot hear.

The correct answer highlights that non-diegetic sound often serves to enhance the emotional tone or atmosphere of a scene. This type of sound is intentionally added during the post-production stage to guide the audience’s reactions or to create a specific ambiance that would not exist through diegetic audio alone. Thus, distinguishing non-diegetic sound as background music added for effect emphasizes its role in storytelling beyond the characters' immediate experience.

Although the other options reference aspects of sound design, they either mix up the definitions or do not accurately define the terms involved. The answer correctly identifies a key characteristic of non-diegetic sound, aligning it with its function in film.