The correct answer is D: Después, nosotros abrimos los libros de química. This means "afterwards, we opened the chemistry books". Abrimos is the first conjugation of the plural of the verb "abrir". The first option, "abren" would be correct if instead of "nosotros" (we), there was a "ellos" (they). Abres would be correct for a "tu" (you), and abre would be suitable for a "él o ella" (He or She).
