Final answer:
The Third Crusade was primarily caused by the Seljuks' recapture of Jerusalem from the crusaders in 1187, leading to the rallying of European leaders to try to retake the holy city.
Step-by-step explanation:
The primary cause of the Third Crusade was C. The Seljuks retook control of Jerusalem from the crusaders. This loss of Jerusalem in 1187 to the forces of the Muslim leader Saladin was a significant blow to Christendom, prompting European monarchs and nobility to embark upon the Third Crusade in an attempt to reclaim the holy city. The fall of Jerusalem prompted Pope Gregory VIII to call for the Third Crusade, which saw figures such as Richard the Lionheart of England, Philip II of France, and Frederick I Barbarossa of the Holy Roman Empire participating.
Differing motivations such as religious zeal, adventure, land, titles, and the promise of the forgiveness of sins spurred many in the West to join the crusading efforts. While the Third Crusade was successful in capturing some territory and negotiating a treaty that granted Christian pilgrims access to Jerusalem, the city itself remained under Muslim control.
The consequences of crusading continued to evolve, with later efforts seeing reduced fervor and shifting political goals. The crusading movement underwent changes, as illustrated by the events of later Crusades, such as the Fourth Crusade's diversion to attack Christian Zara and its subsequent excommunication by Pope Innocent III.