The central teaching of the qur’an is that a. there is only one god b. humanity is the key to life c. daily prayers are required d. muhammad is the last prophet

the central teaching of the Qur'an is  Tawhid, which means the oneness of God.  To worship the Al-Mighty and not associate partners the God. Call for worship the god Allah with no partner no companion and no son to witness that that Muhammad is his only messenger.  

so the answer is D. Muhammad is the last prophet.

hope I helped    and good day :)