The correct answer is: Aside
It refers to a dramatic device that is used to express what a character is thinking, as it is a difficult thing to do for the author. It is a speech or short comment delivered by a character to the audience in a direct way, the other actors on stage are not able to hear it. The things said stay between the character and the audience.
Of the other possible answers soliloquy is similar but with the difference that it is a longer speech revealing motives, inner thoughts, or internal struggles while the aside reveals hidden secrets or judments.
The word monologue gives us the clue, a soliloquy is not a monologue, In a monologue a character usually makes a speech in front of other characters, in a soliloquy the character speaks to himself.
