In answering that question, I would point that person to gospel language in the New Testament. I think the most theologically accurate way to respond is to point to God the Father.
In the opening chapter of the gospel of Mark, he argues that Jesus is the Son of God. He makes a tie in Mark 1:2–8 to the ministry of John the Baptist. Then he gets to the baptism of Jesus, where he points to God the Father affirming from heaven, “This is my beloved Son, in whom I am well pleased” (Mark 1:11).
A simple declaration that God the Father is the father of the Lord Jesus Christ is a clear, simple, gospel way to point to the truth about Jesus Christ.