Which two parameters are valid networking modes for a Linux container (LXC)?

Hmmm. A and D are not “modes”. They Could be “distracter” options. At first, I thought of these options as well.
However, according to
Oracle® Linux Administrator’s Solutions Guide for Release 6
and supplemented by
https://access.redhat.com/articles/1365153 Networking with Linux Containers and https://ex0ns.me/2015/09/17/lxc-nat/:
B. Bridged is one obvious answer. The LXC.CONTAINER.CONF(5) man page indicates under the macvlan interface: “The accepted modes are private, vepa, bridge and passthru”
C. NAT is not so obvious, but is probably correct because NAT is used to set up a veth bridge. Also, “Linux Containers with libvirt-lxc” indocates: “The default virtual network provided by libvirt operates in NAT mode”