Which of these measures did americans fear was likely to lead to nuclear war with the soviet union?
a. u. s. spy flights over cuba
b. cuban refugees coming to the united states
c. a quarantine of cuba in international waters
d. u. s. demands that soviets dismantle their missiles

The measures which to lead to nuclear war with the soviet union is being shown in the option : C. a quarantine of Cuba in international waters. I choose this one because. There was a possibility of war with Russia if Americans act like this towards the counrty belongs to them. 
Do hope that will help you.