Which of the following  best  explains why the united states sent military aid to south vietnam in the 1950s?

The answer is: to help South Vietnam without committing US troops to a war

Explanation:

But what America didn't predict (or they predict, I believe), is that the interference in South Vietnam would become a conflict. After World War II, France left Vietnam and accepted it's asking for freedom. At that moment, Eastern Europe and some regions of Asia were under the communist influence zone (China introduced a communist regime in 1949). In the case of Vietnam, which was split into two parts (a capitalist, and a socialist/communist one), the problem was bigger. Somehow, the country didn't want to be separated, but be part of an influence zone, in this case, the Soviet zone. The threat of the communism be influent in so enormous territory (practically, a half part of the Asia continent, if we count China, the Korean peninsula, and Vietnam), America sent troops to South Vietnam to avoid an attach with the Soviet Union. Then, a war has begun.

the secretary of state, appointed by the president with the advice and consent of the senate, is the president's chief foreign affairs adviser. the secretary carries out the president's foreign policies through the state department and the foreign service of the united states.