I have to admit, I was totally distracted by the caret, and didn't even consider bitwise ops. Nice distraction.

There are problems with the question, though.

Why is @b-2 declared and never used?

Where is @b-17? Answer B cannot be correct

Answer A is not just a distraction, it's wrong to include the "=2^64" because you are deliberately misleading us. A much better A would have been "16^16 is out of range for the bigint datatype' which [in hindsight] is clearly not true.

Still, a nice question. BOL should be updated.