The best answer of the ones that you offer us is B. In general, Cubans escapeing from Cuba and coming to the United States in these decades came because they wanted to get away from the communist regime led by Fidel Castro.
The Castro regime had come to power in the late 1950s in Cuba. Castro soon imposed a very strict set of rules on Cuba, basing them on ideas of communism. These stifled human rights such as free speech. The rules also destroyed private enterprise in Cuba. Because many Cubans thought that they would be better off both economically and in terms of rights in the US, they tried to leave Cuba and come to the US.
Therefore, B is the best of these answers.