Final answer:
User1 should be assigned the Compliance Administrator role in Microsoft 365 to assign Compliance Manager roles, as it offers the necessary permissions without excessive privileges and adheres to the principle of least privilege.
Step-by-step explanation:
The role that should be assigned to User1 in Microsoft 365 to ensure they can assign Compliance Manager roles to users, while abiding by the principle of least privilege, is Compliance Administrator. This role is specifically designed to manage compliance features within Microsoft 365. It provides sufficient permissions to manage roles within the Compliance Center without granting excessive privileges that come with the Global Administrator role. The Security Administrator role isn't sufficient for managing Compliance Manager roles, and the Compliance Data Administrator role does not align with the needed permissions. The Global Administrator role would give User1 unnecessary access to all administrative features across Microsoft 365, which would violate the principle of least privilege.