Who vetoes bills?

The President has the authority to veto bills passed by Congress, making this answer the correct one. When both the House of Representatives and the Senate approve a bill, it is then sent to the President, who can either sign it into law or refuse to do so by issuing a veto. This power is a vital part of the checks and balances established in the U.S. government, allowing the executive branch to check the legislative branch's actions.

The other options represent different roles in government. A Senator does not have veto power; Senators can propose, debate, and vote on legislation but cannot unilaterally reject bills. The House Speaker, while influential in guiding legislation within the House, also does not possess veto authority. Similarly, the Supreme Court's role revolves around interpreting laws and determining their constitutionality rather than vetoing them. Therefore, the President stands out as the sole figure with the power to veto legislation.