Concept:Paul wrote to Philemon to plead for Onesimus, a runaway slave, and to ask Philemon to forgive him.Explanation:Philemon was a Christian slave owner, and Onesimus had escaped from him.Paul appealed to Philemon to forgive Onesimus and welcome him back as a brother in Christ.The letter's main theme is Christian forgiveness, shown through Paul's request for reconciliation.It teaches that believers should forgive others as they have been forgiven by God.Answer:C. forgiveness