Which IPv6 address block is reserved for 6to4 tunneling?

A.
2000::/16

B.
2001::/16

C.
2002::/16

D.
3ffe::/16

E.
fe80::/16

Explanation:
http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/ip6-tunnel.html#wp1055738
Prerequisites
With 6to4 tunnels, the tunnel destination is determined by the border router IPv4 address, which is
concatenated to the prefix 2002::/16 in the format 2002:border-router-IPv4-address::/48. The
border router at each end of a 6to4 tunnel must support both the IPv4 and IPv6 protocol stacks.
Restrictions
The configuration of only one IPv4-compatible tunnel and one 6to4 IPv6 tunnel is supported on a
router. If you choose to configure both of those tunnel types on the same router, we strongly
recommend that they do not share the same tunnel source.
The reason that a 6to4 tunnel and an IPv4-compatible tunnel cannot share an interface is that both
of them are NBMA “point-to-multipoint” access links and only the tunnel source can be used to
reorder the packets from a multiplexed packet stream into a single packet stream for an incoming
interface. So when a packet with an IPv4 protocol type of 41 arrives on an interface, that packet is
mapped to an IPv6 tunnel interface based on the IPv4 address. However, if both the 6to4 tunnel
and the IPv4-compatible tunnel share the same source interface, the router is not able to
determine the IPv6 tunnel interface to which it should assign the incoming packet.
IPv6 manually configured tunnels can share the same source interface because a manual tunnel
is a “point-topoint” link, and both the IPv4 source and IPv4 destination of the tunnel are defined.