A-He has committed an act of treason. This is the right option. Mordred, Authur's illegitimate son, has committed an act of treason. Arthur is at war in France. He has got convinced to renew the war by Gawain but while Arthur is in France, Mordred takes the chance and declares himself king of England. Arthur gets back to England and takes vengeance over Mordred but gets seriously wounded.
B-He has broken the chivalric code. Mordred has done much more; he has become an usurper.
C. He has betrayed Lancelot. Lancelot is in France.
D. He has allied himself with Lancelot. Lancelot is in France and Mordred is in England. They both have different goals.