Answer:
B. You.
Explanation:
It is the understood subject because the other options should need more information to be put in context and clarify to whom is referred the sentence.
In this case, the sentence look is an order, and when we are in front of orders, commands or suggestions the subject could be left outside the sentence because it is understood that "you" is the subject. That is why it is called: understood subject.
I hope this answer helps you.
