The correct answer is B) a foreign conflict that may result in another long U.S. military commitment.
What is meant by "another Vietnam" is a foreign conflict that may result in another long U.S. military commitment.
That is a common expression to avoid a long confrontation or long and difficult consequences. The quote comes from the tough situation the United States got into during the Vietnam War, where they engaged in a war that was not theirs and many young American men died in vain. So that is why people say "we do not want another Vietnam," that is no need to engage in a difficult situation that is not ours, or too difficult with such a minimum gain.