The correct option is A whom; antecedent - lady
A relative pronoun is a pronoun that refers or relates to a noun that's been mentioned before it. The relative pronoun functions as a pronoun as well as a conjunction, because it joins two clauses, where the noun is mentioned in the first clause and it is replaced by the relative pronoun in the second clause, this noun is called the antecedent. Generally, the relative pronoun is right after its antecedent. In the sentence, the 'lady' is the noun that is replaced by the word 'whom', as it refers to the lady. Hence, 'lady' is the antecedent and 'whom' is the relative pronoun so option A has the correct answers. 'Aunt' is not the antecedent because it's a noun that describes the object (The lady), an object is the person or thing that is acted upon (saw) by the subject (We). Hence option B is incorrect. 'We' is the pronoun that refers to the group of people including the speaker that performs the action "saw", it is the subject as it acts upon "saw" the object. It does not represent 'the lady', hence 'we' is not a relative pronoun, making option C incorrect. Option D is incorrect as well, as D too has 'we' as a relative pronoun and 'aunt' as an antecedent, and both these are incorrect.