Final answer:
The correct option to complete the sentence "a las nueve de la noche" is "Me acuesto", which means "I go to bed" in Spanish.
Step-by-step explanation:
The correct option to complete the sentence "a las nueve de la noche" is Me acuesto. This means "I go to bed".
The verb "acostarse" is reflexive in Spanish, which means that it requires the use of reflexive pronouns. In this case, the pronoun "me" is used to indicate that the action of going to bed is being performed by the subject (in this case, "yo" or "I").
So, the correct sentence would be "A las nueve de la noche, me acuesto."
Learn more about Spanish verb conjugation