The correct answer is D. Though the Constitution dictates that the Vice President is the leader of the Senate, the day-to-day leadership falls to the President Pro Tempore.
Explanation:
President Pro Tempore is a senior member of the United States Senate, the chairman of the Senate in the absence of the President-in-Office, the Vice-President. The Senate elects the President Pro Tempore, who normally is the member of the Senate-majority party who has the longest uninterrupted career as a senator.
President Pro Tempore is third in the presidential line of succession after the Vice President and the Speaker of the House of Representatives.
