The Crusades were military expeditions undertaken by a. european christians who wanted to destroy constantinople
b. muslim armies who sought to conquer Europe
c. European Christians to regain the Holy Land from the Muslims
d. Christian and Muslim armies who joined to fight the Mongols

European Christians to regain the Holy Land from the Muslims

Explanation:

The "crusades" were, indeed, military expeditions managed by the European Christians. Its main purpose was to recover the "Holy Land," the area situated between the Mediterranean Sea and the Jordan River. It was under the Islamic rule that was brought about by Muslim expansion.

Many participants volunteered because this was also viewed as a way of removing one's sin and renewing spiritual relationship with God.

declare war, appoint military officers, sign treaties, make alliances