Final answer:
The lady with whom he sat on the bus got off at Elm Street.
Step-by-step explanation:
The lady with whom he sat on the bus got off at Elm Street.
In this sentence, the correct choice is with whom. 'Whom' is the object of the preposition 'with'. 'With whom' is the formal form of the sentence, and it is grammatically correct. It is important to use the correct pronoun form in this type of construction.