Final answer:
Gerald Ford became president after Nixon's resignation, without having been elected to the office of president or vice president, making him the only person in U.S. history to do so.
Step-by-step explanation:
The answer to the question is d) Gerald Ford. Gerald Ford was appointed to the vice presidency in 1973 after Vice President Spiro Agnew resigned amid allegations of corruption. Ford was appointed vice president by President Richard Nixon, and he later assumed the presidency when Nixon resigned during the Watergate scandal. This made Gerald Ford the only person in U.S. history to hold the presidency without being elected to either the office of president or vice president.