Final answer:
Fidel Castro's letter to Nikita Khrushchev during the Cuban Missile Crisis was a request for immediate military intervention if the U.S. attacked Cuba (Option A).
Step-by-step explanation:
During the Cuban Missile Crisis, a considerable point of tension between the United States and the Soviet Union was the content of Fidel Castro's letter to Nikita Khrushchev. Historically, this letter was a request for immediate military intervention (Option A). It was written at a critical juncture when the possibility of a U.S. attack on Cuba seemed imminent due to the discovery of Soviet missile installations on the island. Castro's letter urged Khrushchev to consider a nuclear first strike against the United States if Cuba was attacked. This communication reflects the high stakes and fear prevalent during the standoff, which ultimately led to negotiations between the U.S. and USSR resulting in the dismantling of the missile sites in Cuba and the withdrawal of U.S. missiles from Turkey.