The correct option is D Caliphs
Successors of Prophet Mohammed's are called Caliphs. A caliphate is an Islamic state under the leadership of an Islamic steward(supervisor or care taker) with the title of caliph. Shia Islam holds that Ali ibn Abi Talib was the appointed successor to the Islamic prophet Muhammad as head of the community. Abu Bakr was the first leader after Muhammad on the basis of election.