At a basketball game last​ night, the two referees were Jim and​ Bob, and we​ don't know​ which, but one of them forgot a whistle.
Identify the correct sentence.

A.
One of the referees left his whistle at home.
B.
One of the referees left their whistle at home.
C.
One of the referees left its whistle at home.
D.
One of the referees left his or her whistle at home.