Well, if we just tell you what to say, it's not your idea, and then your employer might be hiring you based on ideas that aren't yours.
Just be honest and tactful.
Think about the importance of diversity. What's the definition? How might it help the work place? Could it hurt the work place? Is it bad to have a work place that's not diverse? What types of ideas or thoughts might arise from diversity?
If you answer those questions, it might help you better formulate your answer to the original question. =]
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