Why are the stories of king arthur and the knights of the round table called legends?

a.
because they are stories that have been passed down and contain historical facts and are about people who actually lived

b.
because they are true stories that have been passed down and contain many historical facts

c.
because they are nonfiction stories that have been passed down and contain facts about kings and knights

d.
because they are stories that have been passed down and contain some historical facts but are mostly fiction