[susan53]

My first reaction was : incredulous (= surprised) that something has happened; sceptical (= doubtful) that something is true. That's how I think I'd use them and I suspect it's what the test wanted.

But then I checked, and in fact I don't think it is wrong. Maybe it's one of those words which is gradually changing its meaning.

I googled "incredulous about" and got a lot of references including this one from The Times Higher Education website which seems to see the two words as complementary, if not synonymous :

.... summaries of each particular case study are necessarily brief and may leave a sceptical reader somewhat incredulous about the interpretations offered. ...

The difference in meaning still exists of course. Incredulous implies you are surprised or shocked by something as well as disbelieving. It has a sort of "Oh, come off it" feel to it. Whereas sceptical is less emotional, it just implies that you have your doubts.

So in this context (ie talking about benefits), personally I thought sceptical was more likely - to check I Googled both incredulous about the benefits (1 result) and sceptical about the benefits (2,670 results).

This suggests that sceptical and benefits are in fact more likely to collocate (occur together) than incredulous and benefits. But it's a matter of frequency of usage, not of grammar or meaning. Anyone who wants to be incredulous about the benefits of something is free to do so.

Hope that helps.
Sue