According to the most widely accepted interpretation of Pauline theology, what is the primary significance of 'union with Christ' (en Christo) in the context of salvation?

A It primarily signifies a mystical experience of oneness with God. B It primarily signifies a legal declaration of righteousness imputed to believers. C It primarily signifies a moral transformation and imitation of Christ's character. D It primarily signifies participation in Christ's death, burial, and resurrection, leading to a new identity and life.