MDM server assignment is required for enrollment. The Apple Business Manager User Guide states, "For devices to automatically enroll in MDM via Apple Business Manager, they must be assigned to an MDM server. Without this assignment, enrollment will not occur during setup." Option C explains why iPhones fail to enroll while Macs succeed. Options A, B, and D are incorrect as new devices don't need resets, approval is automatic for Apple purchases, and 5G doesn't block enrollment.
Reference:
Apple Business Manager User Guide, "Assign Devices to MDM" section.
Apple Platform Deployment Guide, "Automated Device Enrollment" section.