The correct answer is B. The Allow reparenting checkbox, Child records can be reparented to other parent records after they are created, is unchecked.

A service plan must be associated with one Contact, and users cannot change the Contact after the service plan is created. Since field access and validation rules are not the issue, the likely cause is a master-detail relationship where Allow reparenting is not enabled.

Salesforce documentation states that in master-detail relationships on custom objects, administrators can allow child records to be reparented by selecting Allow reparenting. If that option is not selected, users cannot change the parent record after the child record is created.

A and C relate to master-detail sharing settings, not changing the parent record. D is the opposite of the problem; if Allow reparenting were checked, users could change the parent Contact.

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