Answer: C. He was the current Vice-President when Nixon resigned, which automatically made him the new President.
Further detail:
Gerald Ford was appointed to the office of Vice-President after Vice-President Spiro Agnew resigned in the face of a felony charge of tax evasion.
President Richard Nixon appointed Ford as Vice-President to replace Spiro Agnew. Then Ford became President of the United States when Richard Nixon resigned from the presidency when he was facing impeachment over the Watergate scandal.