Question

Which command would create a valid ipv6 default route?

Answer 1

The default route is a route that allows a router to forward packets even though its routing table contains no specific route to the destination network.
The command that creates a valid IPv6 default route is the following:

ipv6 route ::/0 2001:db8:acad:2::a


::/0 is the prefix and prefix length for a default route is ::/0. This matches any address.