A) When you rename the root domain server you will only have to rebuild the user accounts since the root domain holds the authentication (SAM) database.
B) When you rename the root domain server you will simply rename the child domain also.
C) When you rename the root domain server you will lose the entire network orphaning the child domains.
D) When you rename the root domain server you will not need to do anything else since domain names are inherited the child domains will acquire the new domain name.