creating a pop up box

You can do this 2 ways.

You can make a custom form, so when they click a button, it only shows that targeted question and apon saving it sends them back to the main form.  This is not technically a 'pop-up' but has the save affect and doesn't require any code.

The alternative to to use a JaveScript page that will make a pop-up with the question, then using the api feature to write the result to the page.

Hi Matthew,

Can we show a beautiful pop-up based on the selection of values from the dropdown?

Thanks,
Gaurav

Yes, it the information in the pop-up needs to change you can either use different forms, or one form with some form rules. 

Really just depends on your needs.  But be creative :)