Allow Reparenting in Master-Detail Relationships:

In a master-detail relationship, the child record (in this case, OrderItem) is tightly bound to the parent record (Order).

By default, child records cannot be reassigned to a different parent record in master-detail relationships. However, enabling the"Allow Reparenting"option on the master-detail field allows the child record (OrderItem) to be reparented to a different master record (Order).

This is the correct solution as it ensures that OrderItem records can be reassigned to a new Order without changing the structure of the relationship.

Why not the other options?

A. Add without sharing to the Apex class declaration:

Adding without sharing modifies sharing rules but does not affect the ability to reparent child records. This is unrelated to the issue described.

B. Change the master-detail relationship to an external lookup relationship:

Changing the relationship type is not necessary. Additionally, doing so could introduce unintended side effects, as master-detail relationships have features like roll-up summaries that are lost with a lookup relationship.

C. Create a junction object between OrderItem and Order:

Introducing a junction object is not required. This adds unnecessary complexity and deviates from the intended use of master-detail relationships.