In the book of Philemon, what is Paul's primary appeal regarding Onesimus, the runaway slave?

A That Philemon should sell Onesimus and donate the money to the poor. B That Philemon should punish Onesimus as an example to other slaves. C That Philemon should grant Onesimus his freedom and welcome him as a brother in Christ. D That Philemon should send Onesimus back to Paul to serve him in prison.