Final answer:
The share object My Custom Object share is not available/created when My Custom Object is a master-detail relationship object.
Step-by-step explanation:
In the scenario where My Custom Object is a master-detail relationship object, the share object My Custom Object share is not available/created. When My Custom Object has public sharing settings, the sharing settings determine how the object is shared, but the share object is still available. When My Custom Object is not enabled for sharing or is a standard object, the share object is not available/created.