Question

What indicated that the United States had been secretly involved in Vietnam before its official involvement?

Answer 1

The answer to the question stated above is letter b.the Pentagon Papers

The Pentagon Papers which is  officially titled as United States – Vietnam Relations, 1945–1967: A Study Prepared by the Department of Defense, is a U.S. Department of Defense history of the United States' political-military involvement in Vietnam from 1945 to 1967.
The papers revealed that the U.S. had secretly enlarged the scale of the Vietnam War with the bombings of nearby Cambodia and Laos, coastal raids on North Vietnam, and Marine Corps attacks, none of which were reported in the mainstream media.

Answer 2

The correct answer for the question that is being presented above is this one: "b.the Pentagon Papers." The Pentagon Papers indicated that the United States had been secretly involved in Vietnam before its official involvement.