Answer: A. Phrase
Step-by-step explanation:
A sentence is a complete thought. So, it can't be that. A question is a sentence where one is asking something, so that is also going to be a complete thought. A clause is part of grammatical organization and can be both complete or non-complete depending if you have an independent clause or a dependent so, I wouldn't choose that one. A phrase is non- complete thought and it forms part of a clause. So, phrase is your answer.