In the Parable of the Laborers in the Vineyard (Matthew 20:1-16), what is the underlying theological point Jesus is making about God's grace?

A God's grace is earned through hard work and dedication. B God's grace is distributed equally to all regardless of their efforts. C God's grace is a reward for those who start serving him early in life. D God's grace is a sovereign gift, and God is free to bestow it as He wills.