The correct answer is B, "the domino theory". American president Dwight Eisenhower (1890-1969) coined the term in a 1954 speech where he explained the importance of Vietnam for "the free world".
He used the expression to argue that if South Vietnam became communist, this would generate a chain reaction and many near countries would also become communists. It was a classic Cold War idea: this war was fought in the manner of influencing zones.