Final answer:
The correct answer is C. The President Pro Tempore. The President Pro Tempore takes the place of the presiding officer in the Senate if he or she is unable to preside over a meeting.
Step-by-step explanation:
The vice president of the United States is also the president of the Senate. However, when the vice president is unable to preside over a meeting, the Senate elects a president pro tempore. This position is usually held by a senior senator of the majority party and they preside over the Senate in the absence of the vice president.
For example, if the vice president is attending to official duties outside of the Senate or is temporarily unable to act, the president pro tempore takes over the role of presiding officer.