Do you consider yourself smart or hardworking?
Interview questions often do not have one right answer. A question like this is likely an exercise to see how you define terms, whether you can communicate the explanation for both, and arrive at a conclusion. In legal memos and briefs, you cannot just provide your conclusion, so don't simply answer that you are smart. Take the time to explain the logic and reasoning behind your answer.
"I would consider myself both smart and hardworking, but I see a difference between them. Being smart is a result of going through school, learning new things, and meeting expectations for grades and other requirements. It's showing that you can check off the boxes and complete tasks. Being hardworking, on the other hand, is an internal characteristic that I'm not sure can be taught. It is the way that a person approaches life. That's not to say that it guarantees success or that someone who is not hardworking will not achieve the same things that you do, but if you are hardworking, you are easily motivated from within and do not need much external encouragement. Interestingly, I believe that a hardworking person can become smart, but a smart person is not necessarily hardworking."