I am assuming that they are asking about whether or not they can be sued for saying that the books "contain 100% questions" even though maybe only 90% of the questions of the real exams were included.
The answer is that you can, in theory, be sued for anything, but not necessarily successfully.
The people most likely to sue you would be people that purchased the book (most likely because they felt it would be hard for them to pass the exam without buying the book) and were dissatisfied with it (perhaps because they did not end up doing well on the exam). Such people might not have an easy time beating you in court either.
When I read the sentence "Contains 100% Questions of GRE exam from 1978 to 2019", I need to ask myself, what is a "100% question"? Is there some category of questions called "100% questions" and this book contains some of those (maybe 10% of them)? Or does the sentence mean "Contains 100% of all questions ever asked on the GRE exam between 1978 and 2019"? If it meant the latter, then why was the word of not included? I, therefore, would not sue you because I am not 100% convinced that I know what you meant in the title of the book.
Furthermore, the cost of starting an action in court (something that usually costs maybe $75) and taking time off work (or studying for exams!) for the court appearances, seems not to outweigh the cost that they "lost" by buying the book itself! However, there are always people that are willing to pay the money and spend the time to take a case to court, with the hopes that they might be able to make a fortune out of it.