Which of these best explains the split between Sunni and Shia Muslims?

Responses
A The total number of Imams that should be allowed within the religionThe total number of Imams that should be allowed within the religion
B Whether or not the Quran is the most sacred text in the Islamic faithWhether or not the Quran is the most sacred text in the Islamic faith
C Differences in the belief about the number of times per day an individual is required to prayDifferences in the belief about the number of times per day an individual is required to pray
D A disagreement over the leadership of the Islamic world after the death of the prophet MuhammadA disagreement over the leadership of the Islamic world after the death of the prophet Muhammad

D A disagreement over the leadership of the Islamic world after the death of the prophet Muhammad