The statements referred by the question are:
a) It convinced the United States to dismantle its nuclear weapons.
b) It proved that a naval blockade was not an act of war.
c) It showed Cuba that communism should be stopped.
d) It brought the world dangerously close to nuclear war.
The correct statement is D. Historians agree the Missile Crisis was the closest the world got to have a nuclear war between the U.S. and USSR. Nothing before or after this came as close to be direct aggression from one of these countries against the other.
Statements A and C never happened: the U.S. has nuclear weapons until today, and Cuba didn't give up on communism.
Statement B doesn't fit the facts around the Missile Crisis. The naval blockade didn't lead to war only because the U.S. was defensive.
