Final answer:
The battle between King Arthur and Mordred in 'Morte d'Arthur' starts because of a misunderstanding, when a knight draws his sword to kill a snake and is mistaken for initiating combat.
Step-by-step explanation:
In Morte d'Arthur, the battle between King Arthur and Mordred begins due to d) A misunderstanding. During the planned parley for peace between both parties, a knight draws his sword to kill an adder.
However, this action is misinterpreted as the commencement of hostilities, tragically triggering the battle. This famous tale is a classic representation of how misunderstandings can lead to disastrous outcomes.