The best choice that describes the difference between a monologue and a soliloquy is: In a soliloquy, a character believes they are alone. In a monologue, a character gives a speech.
To elaborate, a soliloquy is a type of monologue where a character speaks their thoughts aloud, typically when they are alone on stage, revealing their inner feelings or thoughts. A monologue, while it can also be a speech delivered by a character, doesn't necessarily imply that the character is alone; it can be directed to another character or the audience.