Mrs. Smith is a patient _____I have not seen for 3 years.

A) who
B) whom
I answered B

Right.

The correct answer is A) who.

To determine which pronoun to use in this sentence, we need to identify the role it plays in the sentence. In this case, "Mrs. Smith" is the subject of the verb "have not seen." Therefore, we use the subject pronoun "who" to refer to a person in a nonrestrictive clause (a clause that provides additional, non-essential information about the subject).

You could also rephrase the sentence as "I have not seen Mrs. Smith for 3 years." In this case, it is more apparent that "Mrs. Smith" is the subject, making "who" the appropriate pronoun to use.