The correct answer is: D. no article
The word “doctor” is a common noun, which means it refers to any person who has a doctorate degree. In this case, we are not referring to a specific doctor, so we do not need to use an article.
The article “a” is used before singular common nouns that refer to a general or unspecified person, place, thing, or idea. For example, “I saw a doctor today.”
The article “an” is used before singular common nouns that begin with a vowel sound. For example, “I saw an apple today.”
The article “the” is used before singular common nouns that refer to a specific person, place, thing, or idea. For example, “I saw the doctor today.”
In this case, we are not referring to a specific doctor, so we do not need to use an article.