If you visit the senate chamber during a session on an average day, you're most likely to find the session presided over by a. the majority leader. b. a speaker for the house. c. the vice president of the united states. d. a president pro tem.

The correct option is A.
The vice president of the United States of America serves as the senate president and its presiding officer. In the absence of the vice president, the most senior member of the majority party will preside over the senate meeting and designate other senators from the majority party to take the chairs.