Maintaining field-level validation rules and triggers during migration of custom objects requires thorough planning and execution. Exporting these elements along with the objects ensures they are not lost during transition. Configuring the new system to recognize and apply these rules and triggers maintains data integrity. Manually verifying these rules post-migration confirms their correct function. 'D. All of the above, ensuring comprehensive migration and consistent application of data integrity controls for custom objects' covers all necessary steps for successful integration of these critical components. Reference: Salesforce Custom Objects and Data Integrity