The president pro tempore of the senate is

the most senior senator in the Senate.

Explanation:

The presiding officer of the Senate is the Vice President of the United States, who exercises a number of administrative and ceremonial duties. He or she presides over the business sessions of the Senate and votes in cases of a tie vote. When he or she is not present, the President Pro Tempore presides. This officer is usually the most senior Senator from the majority party, who is elected by the Senate.

the answer is 1.. i think