Welcome to your interview.
The question is on your right: Explain Your Hardest Design Challenge. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
You're discussing a project from your portfolio with an interviewer. They want to understand how you think through a difficult product and design problem, especially how you identified the user problem, handled constraints, and judged whether the solution worked.
Pick a project in your portfolio and explain the most difficult design challenge you faced.