Great question! It's also probably one that only you can answer.
I reserve the right to gently recommend you join a different giveaway or join with a different book, but generally speaking you're the one who is going to know your book best.
It may not be a great idea to join a Sports Romance giveaway when your hero only spends a short part of one scene in the entire book playing basketball with a friend. If the book is based around a basketball player, have at it, though!
Some genres are more open than others, like Contemporary Romance or Romantic Comedy, so those would be a good start.
Like I said, you know your book best, so generally I'm going to defer to you at first. If I think something seems off, I'll contact you about it so we can discuss more. If you're not sure about your book, you can always email me in advance or mention something in the notes section of the application form.