Final answer:
A soliloquy is a speech by a character when alone, expressing thoughts and feelings, while a monologue is a speech delivered in presence of others, and a dialogue involves conversation between two or more characters.
Step-by-step explanation:
The communication involving only one person, such as talking out loud to oneself or having a mental "conversation" in one's head, is known as a soliloquy. This term pertains to a character who speaks to themselves, conveying thoughts and feelings, often when they are alone on stage. Comparatively, a monologue is a speech by one individual that is delivered in the presence of others, while a dialogue involves a conversation between two or more characters.
Knowing the roots of these terms can clarify their meanings. The word monologue derives from Greek, 'monos' meaning single, and 'legein' meaning to speak. In contrast, soliloquy has Latin roots, with 'solus' meaning alone, and 'loqui' meaning to speak. Hence, among the options provided, 4) Soliloquy is the correct term for a speech made when alone.