"The Ferengi is afraid to read Klingon books."
Translation:tlhIngan paq laDvIp verengan.
Klingon plural markers are always optional and never required ("always" and "never" are, perhaps, too strong and you'll learn some exceptions later). The word paq could mean "book" or "books". The word paqmey can only mean "books". When you encounter an unmarked noun, you have to rely on other context to indicate whether it is meant to be singular or plural. Sometimes you have to ask if they meant wa' paq or paqmey. But if, for instance, we had already been discussing various Klingon authors, you could easily determine that I meant Klingon books in general and not a single book. Sometimes there might also be other grammatical clues. The prefix lu- indicates specficially that the subject is plural and the object is singular, so tlhIngan paq lulaDvIp verengan can only mean, "The Ferengi are afraid to read the Klingon book," talking about multiple Ferengi and one book. Thus if we have already been talking about mutiple Ferengi, you are more likely to assume verengan in this exercise is multiple and thus the lack of the lu- prefix would mean that paq also has to be multiple.
However, in this course, you are presented with these sentences without any context. If you had been presented with that Klingon sentence, there is no way for you to guess if we had in mind one Ferengi and one book, multiple Ferengi and multiple books, or one Ferengi and multiple books (but you know if can't be multiple Ferengi and one book because of the lack of the lu- prefix). Thus we accept all of those options as possible translations - you do not have to guess which ones are supposed to be plural, we accept all the variations.
In this exercise, since you are given the English sentence first, we know if there are multiple Fernegi (there are not) and if there are multiple books (there are). So you cannot put the plural marker on verengan, but you can put a plural marker on paq, if you wish. Both paq and paqmey are accepted as correct answers on this exercise.