Which best explains why constantinople did not fall until 1453

The statement that [the city was well protected and repelled attacks by Invaders] best explains why Constantinople didn't fall until 1453. Since the people were armed and protected, invaders had several problems in breaking to the city at any point.

the doctrine of nullification recommended that states living inside the union have the one-sided, inalienable ideal to void any law made by the national government that could be esteemed illegal.

the united states was shaped based on a general agreement among its individual states. hence, it is inferred that under the hypothesis of invalidation, since the states are the establishment of the union, they have the ability to referee and disprove unlawful laws.

i believe your answer is d)