Who served as the 17th President of the United States following the assassination of Lincoln?

The 17th President of the United States following the assassination of Abraham Lincoln was Andrew Johnson. Johnson, who had been serving as Vice President, ascended to the presidency on April 15, 1865, after Lincoln was assassinated the previous evening. His presidency was marked by significant challenges, particularly regarding the Reconstruction era following the Civil War. Johnson faced difficulties reconciling the needs of the Southern states with the objectives of the Republican-led Congress, which sought to impose rigorous conditions on the Southern states’ reintegration into the Union.

Other figures in the choices provided represent different periods in U.S. history, but they did not serve as president following Lincoln's death. Ulysses S. Grant became president later, serving from 1869 to 1877, while Thomas Jefferson and James Buchanan served in the early 19th century and just prior to Lincoln, respectively. Their presidencies did not overlap with the immediate aftermath of Lincoln's assassination, thus distinguishing Johnson as the only correct answer in this context.