C. Cuba's communist leaders demanded Soviet military support after U.S.-trained soldiers had invaded Cuba.
The invasion of US-trained soldiers occurred in 1961 -- The Bay of Pigs Invasion. Cuban exiles trained by the USA's Central Intelligence Agency aimed to overthrow the communist government of Fidel Castro in Cuba, but failed.
Cuba requested that the USSR, as its communist sponsor, place missiles in Cuba to deter further US-sponsored action against the Castro regime. That request, coupled with the fact of American missiles that were located in Europe, within striking distance of the USSR, prompted Nikita Khrushchev of the USSR to send the missiles to Cuba.