Answer:
B is correct because the President pro tempore of the Senate is a senator who presides over the Senate in the absence of the Vice President of the United States, who serves as the President of the Senate.
Option A, "the next in line, after the vice-president, for the presidency of the United States" is incorrect because the correct answer to that would be the Speaker of the House of Representatives, who is fourth in the line of succession after the Vice President.
Option C, "the majority leader of the Senate" is incorrect because the majority leader is a different position in the Senate, who leads the majority party in the Senate and is responsible for setting the agenda.
Option D, "the chairman of the Rules and Administration Committee" is incorrect because the chairman of the Rules and Administration Committee is a committee position in the Senate and does not hold the position of President pro tempore.