According to the Apostle Paul in his letters, what is the primary significance of baptism 'for the dead' mentioned in 1 Corinthians 15:29?

A A ritualistic cleansing of deceased individuals' sins B A symbolic act of solidarity with deceased believers who had not been baptized C A metaphorical representation of the believer's death to sin and resurrection to new life in Christ D A practice intended to convert unbelieving family members who had passed away