It is indeed a rather vague question. That doesn't mean it's a bad question, though - it encourages us to think and to use our imaginations.
I got the right answer by sheer luck because I picked the only one of the listed functions whose mandatory parameter list (excluding parameters shifted to an OVER clause) couldn't be counted on the thumbs of one hand, since I hadn't a clue what sensible crtiteria (if any) could be used to select the odd one out - and after reading the explanation I still can't (because using the definition of deterministic won't work, one has to learn, parrot-fashion, the list).
Indeed, the whole deterministic/nondeterministic thing seems to be something of an awful kludge whose purpose rather unclear. For example CHECKSUM(*) is nondeterministic, despite the fact that it will always return the same value whenever called unless the state of the database has changed between calls - i.e. it fully satisfies the definition given in the BoL page for a deterministic function, but it counts as indeterministic for some unspecified reason, perhaps because someone thinks * is a bad thing, or perhaps because having a column containing CHECKSUM(*) in an indexed view would require some (fairly trivial, actually) code to keep the view corect when columns are added to and/or removed from the underlying tables. I suspect the purpose has by now been perverted to be to avoid some rather boring coding problems in implementation, although it probably started out as a genuine attempt to ban nondeterminism in contexts where it would be silly, and that this perversion is why it has become a kludge with no relation at all with its own definition or with the normal use of the word deterministic.