Salesforce Exam PDII Topic 2 Question 62 Discussion

A software company uses a custom object Defect_c, to track defects in their software, Defect__c has organisation-wide defaults set to

private Each Dafect__c has a related list of Reviewer_c records, each with a lookup field to User that is used to indicate that the User will review the Defect_c.

What should be used to give the User on the Reviewer_c record read only access to the Defect_c record on the Reviewer_c record?