100% is not the answer! It actually varies widely from route to route. For example, a route with a lot of high-yield business traffic can be profitable with a much lower load factor, say 80% than a leisure-oriented route with almost no business traffic and where the rest of the pasengers are highly-price-sensitive. We know the answer is not 100%, since this means that the airline probably COULD have sold one more seat, so the pricing of the overall flight was too cheap.