To which destination will a route appear in the Linux routing table after activating IPv6...

 A route to the destination fe80::/10 will appear in the Linux routing table after activating IPv6 on a router’s network interface, even when no global IPv6 addresses have been assigned to the interface. This is because fe80::/10 is the prefix for link-local addresses in IPv6, which are automatically configured on any interface that supports IPv6. Link-local addresses are used for communication within the same link or subnet, and are not routable beyond the local network. Link-local addresses have the format fe80::interface identifier, where the interface identifier is usually derived from the MAC address of the interface. Link-local addresses are useful for purposes such as neighbor discovery, router advertisement, and address autoconfiguration. Every IPv6-enabled interface must have a link-local address, even if it also has other types of addresses.

The other options are not correct. 0::/128 is the unspecified address, which is used as a placeholder when no address is available. It is not a valid destination for a route. 0::/0 is the default route, which matches all destinations. It is not automatically added to the routing table when IPv6 is enabled on an interface. fe80::/64 is not a valid prefix for link-local addresses, as it is too specific. Link-local addresses use the prefix fe80::/10, which covers the range from fe80:: to febf::. 2000::/3 is the prefix for global unicast addresses, which are routable on the public Internet. They are not automatically configured on an interface, unless stateless address autoconfiguration (SLAAC) or DHCPv6 is used.

References:

IPv6 Link-Local Addresses - PacketLife.net

IPv6 Addressing and Basic Connectivity Configuration Guide, Cisco IOS XE Gibraltar 16.12.x - IPv6 Addressing [Cisco IOS XE 16] - Cisco

IPv6 Addressing and Subnetting