The primary cause of the split between Sunni and Shia branches of Islam is C: The question of who should lead Muslims.
Following the death of the Prophet Muhammad in 632 CE, a disagreement arose over the rightful successor to lead the Muslim community. Sunni Muslims believed that the leader, or caliph, should be chosen from among the community's members, which led to the selection of Abu Bakr as the first caliph. In contrast, Shia Muslims believed that leadership should remain within the Prophet's family, specifically through his cousin and son-in-law Ali. This fundamental disagreement over leadership is the central issue that contributed to the division.