I see what you mean Chris, I would have thought it was ok and not noticed but I'm not the best when it comes to grammar, maybe we need an English expert to comment
The longer I ponder, the more I think the setter is correct.
Prone to, and inclined to, mean the same.
therefore prone, and incline, should be the same?
I'm afraid I didn't, and still don't see anything wrong with 'prone' = 'inclined'. I don't think the 'prone to' is necessary. Chambers itself defines 'prone' as '...inclined...'.