Since Case and Defect__c have Private organization-wide defaults (OWD), users need explicit access to the parent records to access the child records in a master-detail or junction object relationship.
The Case_Defect__c is a junction object between Case and Defect__c.
Option C: Share the parent Case and Defect__c records.
Correct Answer.
To access a record on a junction object (Case_Defect__c), a user must have access to both parent records.
Sharing both the Case and Defect__c records with the user will grant them access to the Case_Defect__c record.
Sharing only the Defect__c record is not enough; access to the Case is also required.
Option B: Share the parent Case record.
Insufficient.
Option D: Share the Case_Defect__c record.
*Not Possible Directly.
Sharing rules cannot be applied directly to junction object records in a master-detail relationship.
Access is controlled through the parent records.
Conclusion:
To share a specific Case_Defect__c record with a user, you need to share both the parent Case and Defect__c records, which is Option C.
Reference:
Sharing Considerations for Junction Objects
Sharing Behavior
Incorrect Options:
Option A: Share the parent Defect__c record.
Insufficient.