Answer: share both parent records, Case and Defect__c.

This is the same question you asked earlier. Your options are garbled by OCR, so I can’t be sure which letter is which:

B reads “Share the parent Case and ... record”, so it is most likely the “both parents” option.
A is garbled (“Cast record Defect_c records”). It could also be a version of “both”, so please check the original wording.
C (Defect__c only) and D (the junction record itself) are clearly the wrong ones.

Case_Defect__c is a junction object with two master-detail relationships, one to Case and one to Defect__c. A detail record has no sharing settings of its own and inherits access from its parents. With two masters, and Private organization-wide defaults on both, the user needs access to both parents to see the junction record. Sharing only one of them isn’t enough.

The other options don’t fit:

Share the Case_Defect__c record (D): You can’t share a junction record directly. It has no owner and no sharing of its own.
Share only the Case (A or B, if either is single-parent) or only the Defect__c (C): The user would still lack access to the other parent....