Soy Gustavo y tengo el pelo gris y uso lentes. Señora Lourdes, ________ es canosa también.
Answer: usted.
Soy Gustavo y tengo el pelo gris y uso lentes. Señora Lourdes, usted es canosa también.
I'm Gustavo and I have gray hair and I wear glasses. Mrs. Lourdes, you are gray too.
Explanation: "Usted" is the subject pronoun for the formal second-person in the singular. Since the speaker speaks her as "señora Lourdes", he must use the formal, second-person pronoun, usted.