The function of the noun clause <em>that you are ready to graduate </em>is that of the subject of this sentence.
If you are unsure how to solve this, just change the whole noun clause with a pronoun, and it will become much easier: That is very clear to me. What is very clear to me? That.
Or use that question technique with this example: What is very clear to me? That you are ready to graduate.
Answer:
B
Explanation:
The second choice. If I’m wrong cuss me out lol