Final answer:
Azure AD Connect is the service used for synchronizing on-premises directories with Azure Active Directory, serving as the preferred solution for unified identity across systems.
Step-by-step explanation:
The service that exists for synchronizing on-premises directories with Azure Active Directory is Azure AD Connect. This tool is designed to help you keep your on-premises directories and Azure AD synchronized to facilitate a unified identity for users of both systems. Azure AD Connect incorporates all features from preceding directory synchronization tools such as DirSync and Azure AD Sync, and it's the recommended solution to use when you need to facilitate directory synchronization.