The correct answer is D. All of the above.
The office of the president can fall vacant due to resignation, death, or removal.
- Resignation: The president can resign at any time by writing a letter to the Speaker of the House of Representatives.
- Death: If the president dies in office, the vice president becomes president.
- Removal: The president can be removed from office by impeachment and conviction by the House of Representatives and the Senate.
The 25th Amendment to the United States Constitution also provides for the possibility of the president being unable to discharge the powers and duties of the office. In this case, the vice president becomes acting president. If the vice president is also unable to discharge the powers and duties of the office, the Speaker of the House of Representatives becomes acting president. The line of succession then continues down the list of federal officers, as specified in the 25th Amendment.