Final answer:
Following the vice president, the next in line for the presidency is the Speaker of the House of Representatives. This position is succeeded by the president pro tempore of the Senate and then Cabinet members starting with the Secretary of State.
Step-by-step explanation:
The United States line of succession outlines the order in which government officials replace the President of the United States if the presidency is vacated. After the vice president, the next in line is the Speaker of the House of Representatives. This is followed by the president pro tempore of the Senate, and then members of the Cabinet, starting with the Secretary of State.
According to the established order of succession, the correct answer to your question is a. Speaker of the House. If something were to happen to both the President and Vice President simultaneously, the Speaker of the House would take over the presidency. The President pro tempore of the Senate does not become vice president but would be next in line after the Speaker.