Answer:
The answer is D. sentence 4
Step-by-step explanation:
Although in informal conversations we can express ourselves in every possible way we want, sometimes it can come to being misunderstood. Or our thoughts said out loud can not make sense without context linked to it. In grammar such expression we call sentence fragment.
Sentence consists of three things: a subject, a verb and a complete thought.
When we lack some of these things, then we have not a regular sentence, but a sentence fragment.
For example: Because of you.
It seems like a regular sentence by its looks, but we miss the main components. What story stands behind because of you? We do not know.
Regarding all said above, sentence 4 lacks verb and complete thought, so it is a fragment.