With User Enrollment, personal and managed iCloud Drives coexist separately. The iOS Deployment Reference explains, "When a device is enrolled with a Managed Apple ID via User Enrollment and iCloud Drive is enabled, an additional iCloud Drive appears in the Files app, separating personal data (linked to the personal Apple ID) from managed data (linked to the Managed Apple ID)." Option B is incorrect as data doesn't merge, C is false as personal data remains accessible, and D doesn't occur during this process.
Reference:
iOS Deployment Reference, "User Enrollment and iCloud" section.
Apple Platform Deployment Guide, "Data Separation with iCloud" section.