Your question is Validate Model Generalization. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You have trained a model and offline results look strong on the data used during development. Before relying on it, you need a clear way to judge whether that performance is likely to hold on truly unseen data.
How would you validate that a model will generalize well to unseen data?