Final answer:
The appropriate indirect object pronoun to complete the sentence is 'me'.
Step-by-step explanation:
The appropriate indirect object pronoun to complete the sentence is me. Thus, the sentence would be:
"Usted compra calcetines nuevos para mí."
The pronoun mí functions as the indirect object in this sentence, indicating that the socks are being purchased for the speaker.
Learn more about Indirect object pronouns in Spanish