The correct answer is a) Christians were being persecuted by Muslims in the Holy Land.
At the time the Pope Urban II wrote that letter, in 1095, the Muslim Seljuk Empire was conquering the region of Anatolia and threatening the eastern border of the Christian Byzantine Empire. The Byzantines, who were Orthodox Christians, were afraid that the Seljuk could conquer their capital Constantinople and for that reason, they requested help from the Catholic pope. Also, the Christian Europeans were worried because the Turkish Seljuk troops in Anatolia were persecuting Christian pilgrims to the Holy Land and European merchants caravans that were looking for eastern spices.
The Pope Urban II saw this as an opportunity to end conflicts among European kingdoms and to expand its political power to new frontiers. That is why he called to every Christian to unite in the first crusade in order to take Jerusalem and the Holy Land.