Step 1: Observe the tense used in the reply. The verb form is in the present continuous tense, which describes an action happening at the time of speaking.
Step 2: For grammatical consistency, the question corresponding to this reply should also be framed in the present continuous tense. This ensures that both the question and the answer refer to the same ongoing action.
Step 3: Among the given options, only option (D) is correctly formed as a present continuous interrogative sentence. Therefore, it is the correct answer.