The answer is A. There was a submarine from the USSR that believed the United States attacked the USSR. They prepared to launch nukes on U.S soil but all of the commanders needed to enter the key to launch. One commander was unsure if the broadcast was incorrect and refused to enter his key. The close call was heard of by both governments and a détente was signed.
Lets also look at the other options.
B: Tear down the Berlin Wall, because the USSR couldn't be trusted anymore . Although this is a possible option, I do believe A is correct. The wall was torn down when relationship of the east changed with the west.
C: fight several battles in Cuba over the missle sites . The Americans never stepped foot on Cuba and instead blockaded them.
D: break off diplomatic relations with the Soviet Union. This already occurred during the start of the cold war.
I hope this information helps you! :)