Which IPv6 transition model should be utilized when two or more independent transition mechanisms are required?

A.
tunneling

B.
dual stack

C.
service block

D.
hybrid

E.
manual

Explanation:
The hybrid model strategy is to employ two or more independent transition mechanisms with the same deployment design goals. Flexibility is the key aspect of the
hybrid approach in which any combination of transition mechanisms can be leveraged to best fit a given network environment.
The hybrid model combines a dual-stack approach for IPv6-capable areas of the network with tunneling mechanisms such as Intra-Site Automatic Tunnel
Addressing Protocol (ISATAP) and manual IPv6 tunnels where needed.
The hybrid model adapts as much as possible to the characteristics of the existing network infrastructure. Transition mechanisms are selected based on multiple
criteria, such as IPv6 hardware capabilities of the network elements, number of hosts, types of applications, location of IPv6 services, and network infrastructure
feature support for various transition mechanisms.
http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/CampIPv6.html
http://www.ciscopress.com/articles/article.asp?p=1763921&seqNum=2