I agree with many of the comments here - on multiple choice I was marked down for only choosing the option with speciale and not the "unica" one as well. Unless I have missed something I would scrap this question, or replace the unica" option with something else so that there is no confusion.
I'd gotten the impression it tended to get used when the vowels on either side of it were the same, and the rest of the time 'when it sounds better'. sigh