Us president Dwight Eisenhower developed the domino theory based on fear of which possible event

President Dwight Eisenhower formulated the famous "domino theory" when speaking about events in South East Asia. In particular, he referred to French Indochina (Vietnam, Laos, Cambodia), a colonial possession of France, where communist guerrillas were fighting a war against colonial rule. He believed that if French Indochina was allowed to fall in communist hands, it would create a "domino" effect,  that is, other countries would also fall, one after another.  

Explanation:

letter a is the answer

c. free trade can lead to low pay for workers in poorer countries.