The first sentence from "Normans" on Wikipedia: "The Normans were an ethnic group that arose in Normandy from contact between indigenous Franks, Gallo-Romans, and Norse Viking settlers."
I looked into that: he was born Karl Vaillant (mistakenly recorded as Weilland), because his parents weren't married at the time. When they eventually did get married, he took his father's last name, and became Karl Benz. Although he himself preferred to spell it "Carl Benz", there's no evidence he ever had it legally changed. So I really think both should be accepted, and either would make a good answer.
Pupil: I don't know.
Teacher: Correct!
What North African city has the same name as an Oscar-winning movie?
Casablanca