I'm confused about the different uses of "dello" and "dallo" and "sullo"- however, I must say: FINALLY - a multiple choice question that is challenging!
I cannot see the multiple choice question you had here, hence I cannot explain you a lot of stuff. Sorry.
This helped me, maybe you'll find it useful: http://en.wikibooks.org/wiki/Italian/Prepositions
Should I really get it wrong just because I misspelled "squalo" as "squallo"? Normally Duoling is forgiving with typos... Oh yeah, and there's the fact that I used "le ragazze" not "i ragazzi" but that isn't incorrect, is it?
I think RAGAZZE specifically means GIRLS, whereas RAGAZZI can mean BOYS as well as CHILDREN.
But yes, sometimes they are more forgiving with spelling mistakes than others. Not sure what their rules are.
But the thing is, in the Basics section, I was taught that "ragazza" means girl or child, and "ragazzo" means boy. And now they seem to have switched it.