Answer A is incorrect because you cannot have 5 Primary DNS
Servers for the same domain. If you do this, you will create a totally
unmanageable DNS situation, where every DNS Server would have to be configured
as a Secondary, and the risk of inconsistency in the zone database for the
domain is extremely high. This is never done in the real world, and probably is
never done in the fantasy world either. Another problem with this answer is that
downlevel clients cannot directly register their IP addressing information with
a DDNS Server.
Answer B is incorrect because if the single Primary DNS Server
for the domain becomes unavailable, then none of the DNS clients will be able to
dynamically register their IP addressing information. The requests could be sent
to Standard Secondary servers, but the Secondary servers need to forward the
registration request to a Dynamic Standard Primary DNS Server. Since the single
Primary DNS Server is unavailable, no machine will be able to update information
in the zone database. Not even the DHCP Server which is acting as proxy for the
downlevel clients will be able to perform the Dynamic updates. Finally, the
downlevel clients are not able to directly communicate with the DDNS Server.
Answer C is incorrect because although almost everything is in
place, all network clients are not able to directly communicate with the Dynamic
DNS Server. In order for this solution to work, you need to implement a DHCP
Server that will act as a “proxy” for the downlevel clients that are
not able to register their own IP addressing information.