It can be your own definition. You can think the worst guy worth franchise QB money. The worst guy who can lead a team to a Superbowl. Whatever.
The only way to get this answer wrong is to name a player you don't think is a franchise QB, like everyone is doing lol.
You can say "Dak Prescott is a franchise QB but he's flawed because X, Y and Z, and that's why he's the worst franchise QB"
You have missed the point if you say "My answer is Dak Prescott because he isn't a franchise QB."