Answer: The sentence that is not a proposition is C. Sit down.
Explanation: A proposition is a statement that can be judged as true or false. A proposition is typically associated with declarative sentences. Among the sentences provided, the only one that is not a proposition is C. "Sit down" as it is an imperative sentence and not a declarative sentence. The speaker, in this case, is not expressing a statement that can be considered true or false: he/she is giving an order instead.
