An armed group of Jews captured a mountain fortress called and defended it for two years from a Roman army that outnumbered them 15 to one.

Masada.

Explanation:

Masada is a fortress which was captured by the Jews during the siege of Masada and defend the fortress for 2 years with less army. The roman army was fifteen times greater than Jewish army but the courage and bravery of the Jewish army did not allowed them to capture the mountain fortress of Masada. The Jewish army defend the fortress with less number was a great achievement.

it should be d or b but i'm sure it's d