A for me
Doing this we will be able to know for every Order, there are the Shipement have been sent.
In the Order page, in the Related list, we will have all the Order Lines and all the Order Shipment Groups.

C works too, but more complete

Correct answer is C

1. Here we have 1 order__c to multiple order_line__c, so the line is a child (lookup or detail side) to order parent object.
2. One or more order_lines can be sent to a specific order_shipment_group (multiple lines to 1 shipment). So again, order_line is a child to order_shipment_group__c parent.
3. Order_line__c acts as a junction object between order__c and order_shipment_group__c.


/ - orderLine1 - \
/ - orderLine2 - - orderShipmentGroup1
order1 - orderLine3 - /
\ - orderLine4 - - orderShipmentGroup2
\ - orderLine5 - - orderShipmentGroup3

I think its C