A

To solve the question regarding the transition associated with characteristic X-rays, we identify the relevant transition energies based on atomic structure. Characteristic X-rays are produced when an electron transitions between specific energy levels (or shells) in an atom. The most common transitions resulting in characteristic X-rays include transitions from higher energy levels (e.g., n=2 or n=3) to lower energy levels (such as n=1). Option A indicates a transition from a higher energy level to a lower one, which is characteristic of K-alpha (K\alpha) emissions. Therefore, the correct answer is A.