. . . . . . . . Mrs. Smith, who introduced us last evening, is my aunt.

The correct answer is D. no article.

The article "a" or "an" is used before singular nouns that refer to a general or unspecified person or thing. In this case, Mrs. Smith is a specific person who has already been introduced, so we do not need to use an article.

The article "the" is used before singular nouns that refer to a specific person or thing that has already been mentioned or that is unique. In this case, Mrs. Smith is a specific person who has already been mentioned, so we could use "the" if we wanted to emphasize that we are talking about a particular Mrs. Smith. However, it is not necessary to use "the" in this case.

No article is used before singular nouns that refer to a general or unspecified person or thing. In this case, Mrs. Smith is a specific person who has already been introduced, so we do not need to use an article.