A state can be defined as a 'political community' which gathers under one leader and abide by a specfic set of rules and laws.
The early states as those seen in the Sumer, where city-states. Small in size, ruled by a king with the support of the local people and land lords.
The ruler also had a small army to maintain local order and to protect against foriegn invasions.
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.
I would have to go with Completion of the transcontinental railroad in 1869
D. All answers are correct.