The forte music element is when the note is high or loud. Meaning the Dynamic markings.
Explanation:
The two basic dynamic indications in music are: p or piano meaning "quiet" or “soft” , forte meaning "loud or strong".
Them would be the correct pronoun for this sentence because books is plural and is an object, so its pronoun must be third person. The 3rd person plural pronoun in English is they, and its objective/accusative form (the form used when the item is the object of a verb) is them, so there's your answer.
I think that the answer would be C. Three
Im not 100% sure though, but I hope this helps!