To meet the given requirements, we need a relationship that allows:
Calculating the total amount on an Order.
This can be achieved using roll-up summary fields on the Order object to sum up values from its Line Item child records.
Roll-up summary fields are only available on the master object in a master-detail relationship.
Reference:
Calculating the line amount for each Line Item based on quantity selected and price.
This calculation can be handled using formula fields or triggers on the Line Item object.
Moving Line Items to a different Order if a Line Item is not in stock.
This requires the ability to reparent the Line Item to a different Order.
In a master-detail relationship, you can enable reparenting by selecting "Allow reparenting" in the relationship field settings.
Given these considerations, the appropriate relationship is:
Line Item has a re-parentable master-detail field to Order.
Option D satisfies all the requirements:
Master-detail relationship allows roll-up summary fields on Order.
Reparenting is enabled, allowing Line Items to be moved to a different Order.
Line Item calculations can be handled on the child object.
Incorrect Options:
Option A & C: Lookup relationships do not support roll-up summary fields.
Option B: If the Order has a master-detail field to Line Item, it contradicts the natural parent-child relationship.
Conclusion:
Option D is the correct choice as it supports roll-up summary fields, allows reparenting, and aligns with the parent-child relationship needed.