Why Re-Parentable Master-Detail Field?
A master-detail relationship allows Line Items to roll up totals (e.g., calculate total amount) to the Order.
Setting the relationship asre-parentablesupports moving Line Items between different Orders.
Why Not Other Options?
B). Lookup field: Does not support roll-up summary fields for calculations.
C and D: Relationships in the opposite direction (Order to Line Item) do not support the required logic.
References:Master-Detail Relationships:https://developer.salesforce.com/docs/atlas.en-us.object_reference.
meta/object_reference/relationships_master_detail.htm
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