"in the sofa" or "on the sofa"?

I think your exercise book might be mistaken becasue im sure that it is "on the sofa". When we say we are "in" something that refers to something like a location, an establishment, or somewhere in which you find yourself inside the object or place in question. However "on" refers to us being on top of something in the sense of which youre referring. Example I am on the whale. Means that i am on top of the whale. I am in the whale. Means that i am inside the whale.

On the sofa is right because no one cat sit inside the sofea.Isn't it? you can sit only on the sofa. so on the sofa is the right option.

I always say on the sofa. What do you mean by excercise? Well, I guess you could say that, but it sounds really weird to me. It reminds me of the talking chair, Chairy, on PeeWee's Playhouse.