The answer seems indeed to be letter B) supernatural event.
That was one of the key elements in medieval romance: deriving mystery and suspense from supernatural elements. Be it a supernatural setting, a supernatural quality of a character, or a mystical creature, some sort of "out-of-this-world" element would always be present in such type of literature.
In the excerpt we are analyzing here, a myterious hand comes out of the water, grabs the sword that was thrown, and then disappears back under the water with it. What sort of being - possessing what is obviously a human hand, since it is not described as being some other sort of hand - would be able to live under water? Certainly, this "person" has supernatural characteristics, someting beyond what regular human beings would be able to do.
Morte D’Arthur by Sir Thomas Malory tells the tales of King Arthur and the Knights of the Round Table.
