Thank you so much Richard and alexwlh. So there must be mistakes in the answer key of the book. This book, as I once mentioned, contains so many mistakes. Not a good book at all!
By the way, as I know, even in British English, when the final r goes with an initial vowel, then the r is pronounced as in sentence 3 ( hour and). I think that's is why the answer says there is a /r/ there. But as you listened, there is no /r/ heard there.
Once again, thank you for your very sharp ears Richard and alexwlh .