Comprehensive and Detailed 150 to 250 words 13of Explanation:14

In a Private sharing model, access is strictly limited to owners and those grant15ed access through specific sharing mechanisms. Here, the requirement is to grant access based on a User lookup field residing on a child record (Reviewer__c) to the parent record (Defect__c).

Criteria-based sharing rules (B) are ineffective here because they can only evaluate fields on the record being shared (Defect__c) and cannot "look down" at values in related child records to determine access. "View All" (A) is too broad as it would grant the user access to every defect in the system, violating the private security model.

Apex managed sharing (D) is the correct choice. Because the relationship between the assigned reviewer and the defect is dynamic and based on a separate object, a developer can write an Apex trigger on the Reviewer__c object. When a reviewer record is created or updated, the trigger programmatically inserts a record into the Defect__Share table, granting 'Read' access to the User specified in the lookup field. This provides the precision required to ensure that only the designated reviewers can see specific defects, maintaining the integrity of the Private OWD while automating the necessary exceptions.

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