The main difference between a monologue and a soliloquy is that a soliloquy is delivered by a character who assumes they cannot be heard by others, revealing their innermost thoughts and feelings. In contrast, a monologue can be delivered to an audience or other characters and often serves to advance the plot or provide information.
Based on the responses you provided, the correct answer is:
In a soliloquy, the speaker assumes they cannot be heard.