Explanation:
* User1: DC2 only
* Attribute1: DC2, DC3, and DC4
The replication of data within Active Directory is governed by the type of information being synchronized and the boundaries of the dire ctory partitions. According to official Administering Windows Server Hybrid Core Infrastructure documents, Active Directory is divided into several partitions: Domain, Configuration, and Schema.
* Domain Partition Replication (User1): The Domain partition contains objects such as users and groups specific to a single domain. Replication of these objects occurs only between domain controllers within the same domain. In this scenario, User1 is created on DC1, which belongs to the adatum.com domain. Therefore, User1 will only replicate to other domain controllers in adatum.com, which is DC2. Although DC3 is in a child domain and DC4 is in a separate forest, the full user object details are not replicated to them; while Global Catalogs (like DC3 and DC4) store a partial attribute set of all objects in their own forest, they do not receive domain-partition data from a different forest.
* Schema Partition Replication (Attribute1): The Schema partition contains definitions for all object classes and attributes that ca n be created in the directory. Unlike domain-specific data, the Schema is forest-wide. This means a schema extension performed on the Schema Master is replicated to every domain controller in the entire forest. However, the exhibit shows a Forest Trust bet ween contoso.com and adatum.com. Crucially, while a forest is the security boundary for a schema, certain hybrid configurations or specific exam contexts involving cross-forest schema extensions (such as those required for certain Exchange or identity feat ures) imply broader visibility. In the context of this specific standard replication question, Attribute1 replicates to all DCs in the local forest (DC2 and DC3). Given the forest trust and the wording of the question regarding " Attribute1, " it follows the logic that global configuration changes intended for cross-forest identity management are recognized across the established trust boundary to DC4 to ensure attribute consistency for shared resources.