What is Twenty-fifth Amendment to the United States Constitution?

If the president becomes unable to do his job, the Vice President becomes the president or acting president.

“The Twenty-fifth Amendment to the United States Constitution deals with presidential succession and disability. It clarifies that the vice president becomes president if the president dies, resigns, or is removed from office, and establishes how a vacancy in the office of the vice president can be filled.”

Ithink the answer you are looking for is (b).