Answer:
D. Mordred tried to steal the English throne.
Step-by-step explanation:
There are some reasons why king Arthur didn't trust Mordred. One of the main reasons is that Mordred tried to steal the English throne. The other one is that Mordred tried to make Guenevere his wife and he wanted to kill King Arthur. So when King Arthur realized that Mordred was against him he couldn't trust any more in Mordred.