Answer:
Move the computer object of the domain controller in Active Directory Sites and Services to SiteA
Explanation:
Changing of the IP address of domain controller to 10.1.100.250/16, this means that address have been assigned an address under the subnet mask 10.1.0.0/16. Among the three site, its site A that has that subnet.
Hence, what you do is Move the computer object of the domain controller in Active Directory Sites and Services to SiteA.