Final answer:
The unique power held only by the U.S. Senate is the ability to approve presidential appointments and treaties. This significant authority serves as a check on the executive branch and is an integral part of the Senate's responsibilities.
Step-by-step explanation:
The power that is held only by the United States Senate is C) Approve presidential appointments and treaties. The U.S. Constitution grants the Senate the exclusive authority to ratify treaties and to give consent for the president's appointments of federal judges, ambassadors, and the heads of executive departments. This is a significant check on the power of the Executive Branch and ensures that the Senate has a say in key government positions and international agreements.
While Congress as a whole has the power to pass legislation, declare war, and levy taxes, only the Senate has the power to provide the necessary advice and consent on presidential appointments and treaties. For a treaty to be ratified or an appointment to be confirmed, a majority vote in the Senate is generally required. However, ratifying treaties specifically calls for a two-thirds vote. These confirmation processes can often become contentious, reflecting the Senate's vital role in shaping federal policy.