The IPv4-compatible method is the closest match when the exhibit requires IPv6 connectivity between servers across an IPv4 portion of the network without using 6to4, 6rd, or ISATAP. IPv4-compatible IPv6 addressing represents an IPv4 address inside an IPv6 address format and was historically used for automatic IPv6-over- IPv4 tunneling between dual-stack nodes. Although this technique is now considered legacy and is not preferred for new designs, it matches the answer choice that directly embeds IPv4 reachability for host-to-host connectivity. ISATAP is normally used inside an enterprise IPv4 network to connect IPv6 hosts over an IPv4 intranet. 6to4 and 6rd are provider or site transition mechanisms that use specific IPv6 prefix behavior and are not simply a direct method between the depicted mail servers. For modern Cisco design, dual-stack or manually engineered tunnels would normally be preferred, but given these choices, IPv4-compatible addressing is the method aligned to the exhibit requirement. Reference topics: IPv6 transition mechanisms, IPv4-compatible addressing, tunneling, dual-stack migration, host connectivity.