Here are the correct responses to your questions:
Question 1: A key reason for U.S. involvement in the Vietnam War in relation to the policy of containment was:
To prevent the spread of communism to other countries.
Question 2: The situation in Vietnam compared to Korea regarding foreign involvement is best described as:
Vietnam was divided into two parts, similar to Korea, with U.S. support for the South.
Question 3: A significant consequence of U.S. military intervention in Vietnam was:
Extremely high numbers of both American soldiers and Vietnamese civilians deaths.