In Spanish the articles and nouns have gender and number, therefore we must identify these two data of the word "libro" in Spanish:
<em>Libro</em> is a singular and male noun.
By this reason, you must use an article that be male and plural for its plural form:
<em>El</em>: Male and singular.
<em>La</em>: Female and singular.
<em><u>Los</u></em>: Male and plural.
<em>Las</em>: Female and plural.
As you can see, the right option is <em><u>"los,"</u></em> by another side, the plural in Spanish regularly is formed adding "s" at the end of the word, in this case, the plural form of <em>"libro"</em> is <u><em>"libros."</em></u>