Final answer:
The social benefit for women in polygynous marriages is the mutual assistance between co-wives in childcare and housework.
Step-by-step explanation:
One social benefit for women in polygynous marriages is that co-wives help each other with child care and housework, which is answer C to the student's question. This form of marriage involves the union of one man with multiple women, where the family unit cooperates to share resources and provide childcare, often with a hierarchy among wives. Berforehand, each wife usually lives in her own house with her biological children, making this cooperative relationship beneficial for managing household duties and child-rearing tasks.