<em>Las pizarras son larg</em><em>a</em><em>s</em>
<em>Pizarra </em>is a fem. singular noun. Generally speaking, fem. singular nouns interact with their corresponding adjectives matching each other when it comes to their quantity and if they are masculine or femenine. So: <em>pizarra larga</em> (singular), <em>pizarras largas</em> (plural); <em>coche azul</em> (singular, masculine), <em>coches azules</em> (plural). Note you add an -es termination when the word does not end with a vowel (<em>azul, azules; lápiz, lápices</em>, and so on. Such rule applies to both nouns and adjectives).
The answer is B. hope that helped
