Good Morning! Here's your answer and explanation in the simplest form I could put it, best of luck!
Answer:
D. menor
Step-by-step explanation:
Translation: Menor = Younger
Translation: Yo tengo quince años; y mi hermana = I am 15 years old; and my little sister is
Because the question is stating that you have a sibling that's 12 years old (Tiene doce) and in the question its asking you to determine how to describe her with her age which we know is 12. In Spanish, what sounds best is "menor" in this case because all the other words are irrelevant to describing her age.
Other answer Translations:
Mayor= Older
Menos= Less
Mejor= Better
^^these have nothing to do with saying she's younger, so D. Menor is your answer!
I hope this helps you! ;)))