Final answer:
The identity of the source is what cannot be kept secret if a confidential source is testifying in court.
Step-by-step explanation:
The correct answer is A) The identity of the source. When a confidential source is testifying in court, their identity cannot be kept secret. This is because the right to confront one's accuser is a fundamental principle in the legal system. It is important for the accused to know who is providing evidence against them in order to have a fair trial.