They aren't wrong answers Matthew. They are strictly correct (although Kwong Ming Hong did mix up Fun and Funny). Fun has become commonly used like that, but it isn't/wasn't strictly grammatically correct. If you are talking to someone under 30-40 in an informal situation Tablo, then yes, you can use that sentence. If you are talking to an older person, or writing for a strict examination, then use 'as much fun' as La Liseuse said.

You might hear some younger people say this in informal contexts, but it's not very good grammar. You should either use an adjective and say "This book is not as funny as the other one", or use a noun and say "This book is not as much fun as the other one." You would use 'as funny as' if you mean 'comical', or 'as much fun as' if you mean enjoyable in some way.

It works. But the proper adjective is funny instead of fun.

I agree with Matthew. You can use this.

I am very surprised that you are receiving so many wrong answers. Your sentence is perfectly correct. "Fun" absolutely can be used as an adjective. "Funny" does not mean the same thing as "fun" at all, it is that same as "comical". https://www.merriam-webster.com/dictionary/fun