"He lost a book"
That would be more likely to mean "He lost the book" but it's not a strict equivalency. (Object prefixes are more likely when the object is animate and specific and less likely when it's inanimate and general, but it's not a hard rule.)
Basically, in this course, they expect you to use the object prefix with people, but not with things ... except for in the lesson about the object prefix, where you're supposed to use it all the time ... which is stupid because when you're strengthening skills, you don't know which lesson you're in, and each sentence should accept all the possible translations regardless of any external context (such as which lesson you're in).
Anyway, the course is missing a lot of valid answers, if you do this lesson again, try it again with "Alikipoteza kitabu" and report it with "My answer should be accepted" and hopefully one day, the course creators will make a decision about it.