The detail from the passage that supports the idea that engagement and focus are connected is this: C. "The glasses would darken as the user became distracted, providing a real-time incentive to stay focused." (paragraph 7)
What is the detail?
The detail that supports the idea that engagement and focus are connected is option C where we see a link between focusing through the glasses and having a clear or darkened view.
When the viewer loses focus, the glasses would darken but if they remain focused by watching through the glasses nothing happens. So, the result of losing focus was darkness and that was an incentive to remain engaged.