Final answer:
The correct answer is option b. ultimately forced Johnson out of the presidential race.
Step-by-step explanation:
In early 1968, increasing opposition to the war within Lyndon B. Johnson's own party ultimately forced him out of the presidential race. Minnesota senator Eugene McCarthy, who called for an end to the war, performed surprisingly well in the New Hampshire primary, receiving nearly as many votes as Johnson.
McCarthy's success encouraged Robert Kennedy to announce his candidacy as well. Facing health problems and declining popularity due to his actions in Vietnam, Johnson decided not to seek reelection and withdrew from the 1968 presidential race.