Which term refers to the audio portion of the video image being viewed on screen?

The correct answer refers specifically to the audio portion that is synchronized with the visuals on the screen. The term used to describe this audio is "Dialogue." Dialogue encompasses the spoken parts of the video, which are essential for conveying the story, emotions, and character interactions.

Natural sound source, while it may refer to sounds that naturally occur in the environment of the scene, does not specifically address the spoken words of the characters. Ambience pertains to the background sound that creates atmosphere but is distinct from the direct speech of characters. Voice-over, on the other hand, represents narration or commentary that is not part of the characters’ spoken dialogue but rather a separate audio element layering over the visuals.

Understanding the context in which these terms are used can greatly help in differentiating between them and grasping the overall audio landscape within a video production.