Final answer:
The correct past participle that completes the sentence is 'había vuelto.'
Step-by-step explanation:
The correct past participle that completes the sentence is había vuelto. The phrase "Si yo hubiera sabido cuál era la norma social, no habría vuelto a casa justo después de la cena." translates to "If I had known what the social norm was, I wouldn't have gone back home right after dinner." Here, the verb "volver" (to go back) is conjugated in the past participle form "vuelto" to match the past tense of the conditional statement.
Learn more about Spanish verb conjugation