Hewwoo :3
What molecule must be removed in order to rebuild ADP into ATP?
In order to answer your question, you must look at the chemical structures of ADP and ATP. ATP has an adenine that goes by 9' nitrogen to 1' carbon (ribose sugar). In turn, the 5' carbon is attached to a triphosphate molecule. You can see that there's no such thing as a water molecule there. Thus, a water molecule was presumably removed during chemiosmotic phosphorylation.
Answer: A. a water molecule