Explanation
Creating a specific Sharing Reason for the custom object is the best way to make sure that users that gained access to a custom object do not lose access to it when its owner is changed. Sharing Reasons are labels that identify why users or groups have access to records. They are used in Apex managed sharing, which allows developers to programmatically create and maintain sharing rules1. Using ''runAS'' system method in Apex classes to enforce record visibility, creating a new record in_Share object with RowCause ''Manual'', and using ''With Sharing'' keyword to make sure record visibility will be considered are not ways to prevent losing access when the owner changes.