The correct answer is: D. no article.
The word "appointment" is a countable noun, which means it can be used with both singular and plural forms. In this case, we are talking about a specific appointment, so we do not need to use an article.
If we were talking about appointments in general, we could use the indefinite article "a" or "an." For example, "I have a doctor's appointment tomorrow."
We could also use the definite article "the" if we were talking about a specific appointment that has already been mentioned. For example, "I have the appointment with Dr. Smith at 3:00 PM."
However, in this case, we are simply saying that we have an appointment at 3:00 PM, so we do not need to use any article.
