Final answer:
In a master-detail relationship between a standard and custom object, the standard object is the parent, signifying that the custom object depends on the standard object for its record's existence.
Step-by-step explanation:
In a master-detail relationship between a standard and custom object within the context of Salesforce or similar CRM platforms, the standard object must be the parent.
This is because the custom object is dependent on the parent for its existence - if the parent (standard object) is deleted, all related detail (custom object) records are also deleted automatically. The master-detail relationship is a tightly coupled relationship where the detail record inherits sharing, security, and other parent characteristics.