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. the vice president of the united states. c. a speaker for the house. d. a president pro tem.

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" since the Vice President only comes in on special occasions. 

The correct answer is A. the majority leader

Although there is the fact that the vice president should be there and preside, the vice president does not do this and actually shows up only when important sessions are in place. This is why the majority leader takes the role of presiding over the Senate since they can propose what should be voted and what shouldn't and similar things.