Final answer:
The Pope took a stand against Bishop Nestorius, affirming Mary's title as the Mother of God, which was a key issue of Christian theological debate during the time.
Step-by-step explanation:
The individual who went against Bishop Nestorius and fought to prove that Mary is the Mother of God was C) The Pope.
During the early Christian debates over Christology, the nature of Jesus Christ and his relationship with God were central to theological disputes. Nestorius, who was the Bishop of Constantinople, proposed that Jesus had two distinct natures (divine and human) and that Mary should only be called the Christotokos (Christ-bearer) rather than the Theotokos (God-bearer or Mother of God).
This view was opposed by Cyril of Alexandria, who argued that Jesus was one person with two natures united and that Mary could indeed be called the Theotokos. The dispute led to the Council of Ephesus in 431 AD, where Nestorianism was declared heretical, affirming the title of Theotokos for Mary. The Pope, along with other bishops, played a decisive role in resolving this theological controversy.