Hi Charlotte -
This can definitely be done! You'll need to create the applicable fields and insert them into a form. Place that form on your unsubscribe landing page, and voila!
I hope this helps!
Like Eric says, create a field (call it LastOptOutReason or similar) and it to the form as radio buttons. But also check with your legal team about requiring any additional action on the user's part to unsubscribe. You may not be able to require a button to be selected before processing the form (so blank/not specified is the default value in Marketo).
Correct, you may not "force" someone to fill in the survey question. So it cannot be required.
And check with legal.