Final answer:
The supporters of Ali in the conflict of who would hold power in the Islamic world during the Middle Ages are called Shia.
Step-by-step explanation:
The supporters of Ali in the conflict of who would hold power in the Islamic world during the Middle Ages are known as the Shia.
The Shia were a group of Muslims who believed that leadership of the Muslim community should reside in the family of Muhammad through his son-in-law Ali. They believed that only descendants of Muhammad should lead the Ummah. In contrast, the majority of Muslims, known as Sunnis, argued that any righteous and competent leader could be appointed as caliph.
Therefore, the correct answer is option b) Shia.