B) Facing impeachment, he resigned from office.
After the Watergate Scandal (June 1972), in which President Nixon was involved, many investigations were carried out so he could be brought to justice. However, two years later, the President decided to resign from office to avoid impeachment. And no sooner had Ford taken office, than he used his newly acquired power to pardon Nixon for any crimes he may have committed. As expected, his decision was harshly criticized and angered many Americans that wanted justice.
When Nixon says " I'm innocent. You've got to believe me (...) If you don't, take my job." he means that if it can not be proven that he was not involved in the scandal, then he'd resign so somebody else would take the Presidency.
