The correct answer is “credibility gap.”
American public began to distrust Johnson administration because of the credibility gap.
During the Vietnam war, skeptic people started to doubt about the U.S. army participation in that war. At first, most Americans supported the sending of troops to South Vietnam, but on March 23, 1965, David Wise wrote an article in the “New York Herald”, and he associated the term “credibility gap” with the Vietnam War. On December 5, 1965, the term was applied again by Murray Marder in an article written for “The Washington Post.”
The other options of the question were b) War Power Act, c) Pentagon papers, and d) 1968 Democratic Convention.