The correct answer is A. King Arthur is mortally wounded.
Step-by-step explanation:
The main idea of a text refers to the main thought or point develop in a text usually through details, examples, descriptions, evidence, among others. In the case of the passage from Morte d'Arthur by Sir Thomas Malory, which is a compilation and interpretation of the text about the life of King Arthur, the author describes the way one character called Sir Mordred, who seems to be the son of King Arthur decided to attack King Arthur with a sword "and right so he smote his father King Arthur with his sword holden in both his hands". Additionally, the attack leads to a serious wound as it "pierced the helmet and the casing of the brain". This means, this passage mainly describes the way Sir Mordred attacked King Arthur and caused him a serious wound that probably caused his death. Therefore, the main idea of this passage is "King Arthur is mortally wounded".