The red roses smell wonderful.
('fantastic' is akin to the words the younger generation uses to describe experiences, like 'awesome'. 'cool', hip', and the perversion of 'excellent' as a cool word. This is not to say it is totally inappropriate to ever use, or to use here, but in this context, it does sound a bit over the top in describing the smell of roses. Perhaps someone's cooking smells fantastic - or some dish you are eating tastes fantastic. Then it is used as indicating very high praise.
Someone has just won a race - He put on a fantastic performance.
Where both the younger and older generations would use the word similarly, is in describing someone as 'looking fantastic' in a new dress or outfit, or with a new hairstyle.
How wonderful the red roses smell.
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