You may find this webpage helpful, http://www.french-linguistics.co.uk/lexcats/determiner.html
There is a bug which sometimes displays the answer to this as "He's a book". This is incorrect - the correct translation is "He has a book". Unfortunately we have no control over this, so please send a bug report and move on. Note that I don't mean the report button in the lesson, which only sends the report to the course contributors who can't do anything about it, but a bug report to Duolingo staff, which can be found at the link above.
You don't, except that in the context of the sentence "he" seems more likely.
Il a un livre = He has a book.
You need "Il" to go with "avoir" so that someone can have the book.
You can't say "C'a un livre" but you could say "C'est un livre" if you wanted to say "It is a book".
Trofaste, I know this is incorrect and I did report it. I have moved on; this is from months ago.
From months ago but it still happens, it's a very long standing issue which has been driving us all crazy for ages... In asking for reports I don't mean the report button in the lesson - that only goes to the contributors in the Incubator who can't do anything about it (much as we wish we could!) - but a bug report that goes to staff (see the link in my other comment). Thanks for reminding me to clarify that!
As Trofaste explained, there is a bug in where it displays 'He's a book' instead of the correct answer written, 'He has a book'. If you're talking about something else, please be more specific. If not, perhaps try to read the current thread before posting.