The correct Answer is C: the U.S. launched a naval blockade of Cuba. President John F. Kennedy had previously met with the Soviet Premier, Nikita Krushchev, at which time the Soviet Leader had browbeat him mercilessly. Believing Kennedy to be weak, he then ordered offensive missiles placed in Cuba aimed at strategic targets within the U.S. In response, President Kennedy ordered a naval blockade of Cuba to prevent the installation of further missiles, or further development of those in place. This, of course, was during the height of the Cold War, and Kennedy did not dare use the word "Blockade," which would have been an act of war. Instead, he called his action a "quarantine," basically a blockade by another name. It was the closest the U.S. came to nuclear war. After a tense several days, the Soviets backed down. They agreed to remove the missiles from Cuba, in exchange for which the U.S. withdrew offensive missiles placed in Turkey aimed at the Soviet Union.

What you are asking about here is the incident known as the Cuban Missile Crisis.  This incident happened in the fall of 1962.  It is widely seen as the closest that the world ever came to war between the US and the USSR.

The Cuban Missile Crisis was touched off when US spy planes detected short and medium range Soviet nuclear missiles in Cuba.  These gave the Soviets (for the first time) the ability to hit the US mainland with nuclear weapons.

In response, the US instituted a naval blockade of Cuba.  This was a dangerous move because it could have touched off a war with the Soviets if their ships had tried to run the blockade and the US had tried to stop them. Therefore, the right answer is C.