I’m sorry, . . . . . . . . doctor is on leave today. A. a B. an C. the D. no article

The correct answer is: no article.

The reason is that the doctor is not a specific doctor, but rather any doctor. We are not referring to a particular doctor, so we do not need to use an article.

The other options are incorrect because they all imply that we are referring to a specific doctor.

• A doctor implies that we are referring to one doctor out of a group of doctors.
• An doctor implies that we are referring to a doctor who is not a man.
• The doctor implies that we are referring to a specific doctor who is known to the listener.

In this case, we are not referring to a specific doctor, so we do not need to use any article.