The correct answer is letter D.
Explanation: Despite Johnson's best efforts, the unrest in the black communities that was causing tension in the country, and the Vietnam War.
Controversy over the war became acute in late March 1968, when the president limited the bombing of North Vietnam to begin negotiations. At the same time, he surprised the world by abandoning his candidacy for his likely reelection so that he could devote himself entirely, without being hampered by politics, to the pursuit of peace.