Answer: C) An arm appears out of the water and shakes the sword three times.
Step-by-step explanation: in a text, a main idea is important information that tells more about the overall idea of a paragraph or section of a text. In the given passage from Morte d’Arthur by Sir Thomas Malory, we can see the description of how Sir Bedivere throws the sword into a lake, and an arm appears out of the water and shakes the sword three times, before vanishing into the water along with the sword. The option that represents the main idea of this paragraph is option C.