The correct answer is C) the Pentagon Papers.
What indicated that the United States had been secretly involved in Vietnam before its official involvement was the Pentagon Papers.
The United States Department of Defense had a secret report about military involvement in the War of Vietnam. They called the Pentagon Papers. Daniel Ellsberg, a military analyst in the case, considered in 1968 that the information should be released to the public. In March 1971, he gave a copy of the papers to the New York Times. The papers showed how previous administrations had misled public information about the involvement of the US in Vietnam.
The other options of the question were A) the 26th Amendment, C) the 1973 cease-fire, and D) Vietnamization.