Why did the US withdraw from Vietnam?

In 1961, after two decades of indirect military aid, U.S. President John F. Kennedy sent the first large force of U.S. military personnel to Vietnam to bolster the ineffectual autocratic regime of South Vietnam against the communist North. Three years later, with the South Vietnamese government crumbling, President Lyndon B. Johnson ordered limited bombing raids on North Vietnam, and Congress authorized the use of U.S. troops. By 1965, North Vietnamese offensives left President Johnson with two choices: escalate U.S. involvement or withdraw. Johnson ordered the former, and troop levels soon jumped to more than 300,000 as U.S. air forces commenced the largest bombing campaign in history.

d) a constitutional amendment

the answer to fill in the blank in this question would be the word resource, it is defined as the money, material or people (since human resource is a very valuable good) that a company or a person has at disposition (meaning they can use the resource as they see fit), to be able to complete a task or achieve a goal.