Answer:
Explanation:
Hmm I actually don't know either. As a native speaker, it puzzles me that the website told you to use この (which I agree is informal), but asks you to use ください which is slightly on the formal side...the style switch is a little jarring. Maybe its just how I talk, though.
But your answer perfectly matches the given outline, so I'm not entirely sure what you got wrong. I'm sorry...I just wanted to let you know that mixing informal and formal tones isn't used as much-and when it is used, it sometimes may come off as a little annoyed.
I know it says to order one, but why not try ordering all of them? It uses up three words perfectly. このラーメン、とんかつ、とすしをください。
If that doesn't work, I usually say "sushi" as "osushi". It's a preference thing tho, so I doubt it'll work. Sorry if this doesn't help all that much!