You are all wrong. And right. There are a couple of hundred different CF mutations. They get stuck in all different places (golgi, degraded, get to surface but isn't quite function, etc, etc) Hence, the many different phenotypes for CF.
Now, the original question should probably be changed to, what is the defect in the most common type of CF which I think is the deletion of one amino acid at number 508. That, someone else will have to look up because I'm to lazy.