Which technique can be used to deploy IPv6 services in an IPv4-only routed backbone network?

NAT64 translates between IPv6 and IPv4 address spaces but does not provide a mechanism to tunnel or carry IPv6 traffic across an IPv4-only backbone; it addresses protocol translation at network edges, not transit.

6PE (IPv6 Provider Edge) requires an MPLS-enabled backbone to carry IPv6 prefixes as labeled routes; it cannot operate over a pure IPv4 routed backbone that has no MPLS label forwarding.

6RD is specifically designed to transport IPv6 over an IPv4-only infrastructure without requiring MPLS. CE routers derive a unique IPv6 prefix from the 6RD prefix and the CE's IPv4 address, encapsulate IPv6 in IPv4 (protocol 41), and forward it to 6RD border relays at peering locations that decapsulate and forward natively into the IPv6 Internet. The backbone remains IPv4-only throughout.

Dual-stack Lite (DS-Lite) does the opposite - it carries IPv4 traffic over an IPv6 access network back to an IPv4 Internet via AFTR, not IPv6 over an IPv4 backbone; NAT44 is also unrelated to IPv6 service delivery.