Physics GRE GR 1777
11. Quantum Mechanics (Characteristic X-rays)
By definition in the Wikipedia, characteristic X-rays are emitted when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is “characteristic” to each element.
Therefore, the answer is (C).
Tips) Actually, I didn’t see this in my QM textbooks. So the only way to solve the problem was to memorize this phenomenon from the Internet search. If I would find the better way to solve the problem, I will post it.