What caused the earliest disagreement between groups that would become Shia and Sunni Muslims

Though the two main sects within Islam, Sunni and Shia, agree on most of the fundamental beliefs and practices of Islam, a bitter split between the two goes back some 14 centuries. The divide originated with a dispute over who should succeed the Prophet Muhammad as leader of the Islamic faith he introduced

answer; ///i believe the correct answer is (b) the writing of the ancients who lived in india after the indus valley inhabitants died out;