A master-detail relationship is a parent-child relationship in which the master (parent) record controls certain behaviors of the detail (child) record, such as security, ownership, and deletion. A re-parentable master-detail relationship allows the detail record to be moved to a different master record, as long as the user has the appropriate permissions. This relationship implementation supports the requirements of calculating the total amount on an Order, calculating the line amount for each Line Item, and moving Line Items to a different Order. A lookup relationship is a looser association between two objects, in which the parent record does not control the behaviors of the child record. A re-parentable lookup relationship also allows the child record to be moved to a different parent record, but it does not support the automatic roll-up summary fields or cascade deletion that are needed for the requirements. Therefore, option A is the correct relationship implementation for the scenario. References: Relationships Among Objects | Salesforce Help, Cert Prep: Platform Developer I:
Data Modeling