b. President Pro Tem
Johnson's impeachment trial in the Senate began on Mar. 13, 1868. Only a single vote prevented him from being thrown out of office.
Having inherited the presidency upon the assassination of Abraham Lincoln, Johnson had no Vice President of his own. As President Pro Tem of the Senate, Sen. Benjamin Wade of Ohio was next in line of succession. Wade was one of the Radical Republicans who were the biggest advocates for Johnson's removal.
The rules of succession have since been changed. The President Pro Tem is now third in line, after the Vice President and Speaker of the House.