Concept:The blank needs a relative pronoun that refers to a person and acts as the object of the verb “visit.”
Explanation:The sentence is: “Is that the lady ______ you planned to visit?”
Here, “the lady” receives the action: you planned to visit the lady.
In formal English, when a person is the object of a verb, the correct relative pronoun is “whom.”
“Who” is used as a subject, “whose” shows possession, and “which” is not used for people.
Therefore, the sentence becomes: “Is that the lady whom you planned to visit?”
Answer:D. whom