The president pro tempore of the Senate is

the most senior senator in the Senate.

the whip of the majority party.

the leader of the majority party.

the senior senator of the majority party.

The president pro tempore of the Senate is the senior senator of the majority party. The correct option among all the options that are given in the question is the last option. He is actually the constitutionally recognized officer of the senate and he is the person who presides over the chamber in case of the absence of vice president.