The AZ-800 content covering Active Directory Sites and Services clarifies that site, subnet, and site-link objects live in the Configuration partition. The guides emphasize that administration of the Configuration naming context is restricted to Enterprise Admins and to Domain Admins of the forest-root domain. In the context of changing replication topology parameters---such as editing the replication schedule on site links---the documentation notes: ''Only Enterprise Admins or administrators in the forest-root Domain Admins group have default permissions to modify site and site-link objects,'' because these objects affect replication forest-wide.

Applying this to the scenario:

ContosoAdmin1 (Enterprise Admins) has forest-wide rights to modify site links.

ContosoAdmin2 (Domain Admins in contoso.com, the forest-root domain) also has the required rights to change site-link schedules.

CanadaAdmin3 (Domain Admins in canada.contoso.com) does not have default permissions in the Configuration partition for forest-wide site-link administration.

Thus, to meet the technical requirement to change all site links to a 30-minute schedule, the users who can perform the task are Admin1 and Admin2.