Welcome to your interview.
The question is on your right: ISR to Thread Safe Queue. 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.
Implement a custom queue structure that can safely pass messages between a high-priority interrupt service routine (ISR) and a low-priority user thread without causing race conditions.