Question 34: Mark the letter A, B, C, or D on your answer sheet to indicate the undelined part that needs correction in each of the following questions: For most of their (A) history, especially (B) since the 1860’s ( C), New York City has been undergoing (D) major population changes.
The main subject of the sentence is: New York City (New York City) => use possessive adjective “its”.
Edit: their => its
=> For most of its history, especially since 1860, New York City has experienced
major changes in population.