The correct answer is letter C.
Fidel Castro declared he was a Marxist-Leninist as far back as 1953, during his guerrilla campaign against Fulgencio Batista ( Cuban dictator).
Despite Castro’s early attachment to Russian communism, the Kremlin didn’t feel the same way towards Fidel.
The official diplomatic relations between the USSR and Cuba took place in May 1960, when the Soviet embassy opened in Havana
The Soviet Union and Cuba had different ideological aspects. Castro wanted to export the Cuban revolution to the rest of Latin America, he believed that revolutions could in any part of the world. But for the Soviet Union, these ideas were against Marxism-Leninism, which believed in certain standards of modernization were necessary for true communist progress.
Against Soviet advice, Cuba made unsuccessful events to set up guerilla in some Latin American countries. Russian criticism of these actions, along with the death of Che Guevara, weakened relations between the USSR and Cuba, and led Castro to openly criticize Marxism.
The Soviet Union also didn’t appreciate some of Castro’s attitude which resulted in the Kremlin cutting back most of the economic support it provided to Cuba.
Fidel’s change of heart towards the Soviet Union happened when he realized the how much Cuba was economic dependent on the Soviet Union. As a reward for Cuba’s restored loyalty, the Kremlin bailed out the Cuban economy, and continued its support until the collapse of the USSR.