The statement that best explains a cause of the Crusades is: "The Byzantine Empire lost some of its territory to the Seljuks, which it appealed to the pope to help get back."
This appeal for assistance was a significant factor that led to the initiation of the Crusades, as the Byzantine Emperor Alexios I Komnenos sought military help from the Western European powers to reclaim lost territories and to counter the threat posed by the Seljuk Turks.