Answer: The Vice President
Step-by-step explanation:
According to Article I, Section 3 of the U.S. Constitution, the Vice President is the President of the Senate, however he or she can not vote in matters of the chamber, such as in the case to approve legislation, unless when there is a tie, that is to say, when there is a 50-50 party split in the Senate.
If the Vice President ever happens to be absent, he or she should be replaced by the president pro tempore, which usually is the longest-serving member from the majority party.