Answer:
C. Pope Leo IX and Patriarch Michael Cerularius excommunicated each other
Step-by-step explanation:
The Great Schism refers to the break of communion between the Roman Catholic and the Orthodox. Mostly happen because of the fight for authority and difference in some key religious teachings. (for example, Roman Catholics require the priest to be celibate while the Orthodox allowed married Clergy)
The situation escalated after Cartularies (prominent figure in the Orthodox) and Pope Lex IX (prominent figure in the Roman church) excommunicated one another due to this difference. Compromise was very unlikely to happen since both of them sincerely believed that they represented the right interpretation of God's teaching.