The correct answer is D. The president pro tempore of the Senate is the senior senator of the majority party.
The president pro tempore of the United States Senate is the second most important position in the Senate and the highest position as senator. It is established by Article One of the Constitution of the United States. The Vice President of the United States is the President of the Senate, this being the highest position of a member of the Senate; during his absence, the President pro tempore chairs the Senate. The President pro tempore is elected by the Senate; by custom it is, typically, the Senior Senator of the Party with a majority in the Senate. The President pro tempore is the fourth person in succession to the President of the United States, after the Vice President of the United States and the President of the House of Representatives of the United States.