Answer: lo
Translation: it
Step-by-step explanation:
Direct object pronouns in Spanish change gender and number depending on the noun or nouns they replace.
Since "el libro" is a masculine and singular noun, the direct object pronoun that replaces it must also be masculine and singular: Lo
Example:
Spanish: He leído el libro. Lo he leído.
Translation: I have read the book. I have read it.
Spymore