Refer to the exhibit. All of these network devices are running IPv6. When host A is attached to this link, how will it discover the presence of routers B and C?

There is no 'ICMP get route request' message type defined in the ICMPv6 specification - this is a fabricated mechanism that does not exist in IPv6.

Routers do not passively monitor all incoming traffic to detect new hosts and then spontaneously push route information - host-to-router discovery is initiated by the host via NDP Router Solicitation, not by passive detection.

NDP includes a Router Discovery function where a newly attached host sends a Router Solicitation (RS) to the all-routers multicast address ff02::2, prompting routers to reply with a Router Advertisement (RA). Routers also send periodic unsolicited RAs to the all-nodes multicast address ff02::1, so host A will receive these announcements from both routers B and C and learn their presence without waiting for a full solicitation cycle.

Gratuitous ARP is an IPv4 mechanism and has no equivalent in IPv6 - ARP is entirely replaced by NDP in IPv6, and no 'gratuitous ARPv6' message type exists.