Active voice makes an action sound like it’s happening at that very moment. It uses less words, most of the time, and does not use words such as was, were, is, etc.
Usually to change a passive statement to an active statement, we can change the order of the sentence. Try to take the verb of the sentence (in this case, operated) and bring it to the beginning, and take the subject being acted upon (in this case, the patient) and place it at the end.
Your answer then would be: They operated on the patient.
