The correct answer is D) Alexius failed to deal with the Venetians and other Crusaders after he enlisted their help.
The event that was the most significant in damaging the strength of the Byzantine Empire during the Crusades was "Alexius failed to deal with the Venetians and other Crusaders after he enlisted their help."
pope Urban II, in 1095, started the Crusades when he ordered to expel the Turks from the Byzantine Empire. It represented the beginning of the "Wars of the Cross." The Holy City of Jerusalem was in control of the Muslims. Western Christian warriors defeated the Muslims in 1099, recapturing Jerusalem, but in 1187, a Muslim ruler named Saladin, reconquer Jerusalem and the Muslims controlled the Temple of Solomon. The event that was the most significant in damaging the strength of the Byzantine Empire during the Crusades was "Alexius failed to deal with the Venetians and other Crusaders after he enlisted their help."