The correct answer is B) The US gave economic aid to countries threatened by communism.
The Truman Doctrine was a law developed by President Harry Truman who wanted to give economic aid to countries who could possible fall under the control of the Soviet Union. At this time, the countries who were in jeopardy of falling to communism were Greece and Turkey. This is why Truman approved giving these two countries $400 million in aid.
Truman took this same idea and applied it to the members of the Southeast AsiaTreaty Organization, giving them money in order to support their economy and prevent communism from spreading.