The best choice that describes the difference between a monologue and a soliloquy is:
In a soliloquy, a character believes they are alone.
A soliloquy typically occurs when a character speaks their thoughts aloud while believing they are alone, often revealing their inner thoughts and feelings. In contrast, a monologue is a longer speech delivered by a character, which may be directed toward other characters or an audience and does not necessarily imply that the speaker is alone.