Here we have two words, unos is an indefinite article that is a determiner used to refer to a particular person or thing when the reader or listener doesn't know which one is being described, that is, when talking about something that is not defined. On the other hand, cuadernos is a masculine plural noun that means books. Therefore, we must find the singular form of the indefinite article and the noun and this is un cuaderno.
You have to look at the endings. They have to be the same, which would leave you with A and C. In A it has the lo (article), which means it. You need the article in this sentence.