Sentence Correction

Realizing that its best hope of defense against an invasion from Persia would be a strong navy, the assembly of all Athenian citizens voted to allow Themistocles to use the surplus silver from the public mines to build 100 ships.
A.its best hope of defense against an invasion from Persia would be
B.its best hope of defense against an invasion from Persia will be
C.its best hope of defense against a Persian invasion would have been
D. their best hope of defense against a Persian invasion would have been
E.their best hope of defense against an invasion from Persia would be

Explanation: D & E are incorrect for using their to refer singular Assembly C- Incorrect for the usage of would have been B- Incorrect for the usage of will be, will show certainty A- correct, usage of would be is correct as we use would for expectations of the future.