Answer: C) A group of words that contains a subject and a verb and that is used as part of a sentence.
Explanation: A clause is basically like a sentence in that it also contains a subject and a verb, just like a sentence. The main difference is that it does not always have an independent meaning like a sentence does. Therefore, it can be used as part of a sentence.
Example of clause: The girl ran away.
This has a subject (the girl) and a verb (ran). It can also be used in a sentence: The girl that I met at school ran away from me.
I hope this helps youu :)