Explanation:
User1:
This is a domain-level object.
Since DC1 and DC2 are in the same domain (adatum.com), User1 will be replicated to DC2.
DC3 is in a different domain (west.adatum.com), but it is in the same forest. Since it's a Global Catalog (GC) server, it will receive a partial replica of the adatum.com domain, including the newly created User1.
DC4 is in a completely different forest (contoso.com) and there is no direct trust relationship between contoso.com and west.adatum.com, so User1 will not be replicated to DC4.
Attribute1:
This is a schema-level object. The schema is a forest-wide object.
The schema master for the adatum.com forest is DC1, so any changes to the schema (such as adding a new attribute) are initially made on DC1.
These changes are then replicated to all other domain controllers in the adatum.com forest, which includes DC2 and DC3.
However, DC4 is in a different forest, so it will not receive the schema changes made in the adatum.com forest.
In summary, User1 and Attribute1 will be replicated to DC2 and DC3