Class D would be a better fit for you. Looking at your online profile I'd guess that you're very roughly 1000 FIDE strength.
first tournament, class D or C?
But I guess if you just want first place then go Class D.
Problem seems to be solved: I am only allowed to enter the D class (i thought that 'open' indicated 'anyone willing can enter").
My next hurdle is that I am only allowed to enter with a Fide ID. I phoned my confederation (no reply) and have send them an email (waiting for reply). The italian organizers are looking if they can do something by their elo liasson officer.
It's hard to convert between rating systems, but typically I find that peoples' blitz ratings are a couple hundred points higher than their USCF rating, which is a couple hundred points higher than their FIDE rating. Obviously Italy won't use USCF, but the end result is the same; your FIDE rating is probably 300-400 points lower than your online blitz rating. Mine is.
You'll get good competition against equal or slightly stronger opponents in class D. I'd be surprised if you won class D, though feel free to prove me wrong!
Also most people have a little trouble in their first tournament, just getting used to the atmosphere and three-dimensional pieces and all that stuff.
Class D has 20 players registered at the moment, all Italian, fide rated from 999 to 1369
Class C has 31 players registered, most Italian, some international, rated from 1306 to 1709.
I don’t have a fide rating yet. Here in chess.com I am rated 1500-ish.
Would you advice me too register for class D or (if allowed) would it be better to opt for class C?