In the book of Philemon, what underlying principle does Paul appeal to when advocating for Onesimus, Philemon's runaway slave?

A The Roman law that granted slaves the right to seek asylum. B The obligation of Christians to abolish slavery immediately. C The transformed relationship in Christ, where both Philemon and Onesimus are now brothers in faith. D The economic benefits that would result from Onesimus's continued service to Philemon.