- Unbalanced force causes objects to move
- A book is a kind of object
- A book on the table starts to move horizontally
- Therefore, an unbalanced force is applied on the book

So, an unbalanced force is applied on the book
Question
If a book sitting on a flat table starts to move horizontally, it is most likely moving because

Context
- Unbalanced force causes objects to move 
- A book is a kind of object 
- A book on the table starts to move horizontally