The main difference between a monologue and a soliloquy lies in the speaker's awareness of their audience.
A monologue is typically a long speech delivered by one character to other characters or an audience, and it can serve to advance the plot, reveal character emotions, and provide exposition. The speaker is aware that others are listening.
A soliloquy, on the other hand, is a speech delivered by a character who is typically alone on stage, expressing their thoughts and feelings directly to the audience. In this case, the speaker assumes they cannot be heard by others and is essentially revealing their inner thoughts, often creating intimacy with the audience.
So, the correct answer to your question would be: In a soliloquy, the speaker assumes they cannot be heard.