Popup dialog window based on selection in radio select list? Use case:

A radio select list with Option A and Option B.

When User selects Option A, a popup appears with specific "A" message to verify the selection before User saves the page.

When User selects Option B, a popup appears with specific "B" message to verify the selection before User saves the page.

Suggestion on how to develop this? Hoping there's a module.
Since the popup appears as an immediate reaction to the user selection, this looks like a job for JavaScript or jQuery. Take a look at the jQuery Dialog user interface. And here is a tutorial on working with JavaScript within Drupal.