Answer:
C. The difference between the truth and what the administration said during the Vietnam War
Step-by-step explanation:
"Credibility Gap"
When the U.S. troops first indulged in the Vietnam war, American people supported the government on its war efforts. As the war becomes long and various news coming out from the war zone about the brutal killing of common people and My Lai massacre the public support decreased.
American citizens start questioning the Johnson administration and believed a credibility gap had developed which makes it difficult to believe in the government.