Koen Verbeeck (10/28/2014)
"only one null " is someway misleading. Precisley it's a "only one null value per column".
If the unique index is not a filtered index 😉
The question was about a unique "key", which I assume is intended to mean unique "constraint" not "index" (given the context with primary key in the same sentence, and the answer option about defauult index created).
One NULL is correct for a single-column constraint, not for composite. A better way to phrase it is that a UNIQUE constraint allows NULLs (as per ANSI standard), but treats NULL as a normal value that also has to conform to the uniqueness constraint (in violation of that same standard).