Answer:
Correct answer is D. He believed his power to rule came from God.
Step-by-step explanation:
A is not correct as before him Christianity wasn't recognized as official religion.
B is not correct as there were many pretenders to the throne, which Constantine managed to defeat.
C is not correct as Constantine accepted Christianity as official religion and even decided to baptize himself.
D is correct as he believed that God protected him and that he allowed him to rule in his name.