The correct answer is "millón" because it is the Spanish word for "million." The other options, "millones," "mil," and "cientos," are also numerical terms, but they do not specifically refer to the number one million. "Millones" means "millions," "mil" means "thousand," and "cientos" means "hundreds." Therefore, "millón" is the most appropriate choice for representing the number 1000000 in Spanish.