What is a default gateway redundancy protocol in general terms (FHRP-neutral), and what problem would occur without one in a dual-router network?

• A.First Hop Redundancy Protocols (FHRP: HSRP, VRRP, GLBP) provide a virtual IP and MAC address shared between two or more routers; end hosts use the virtual IP as their gateway. Without FHRP, if the router a host points to fails, the host loses internet connectivity until its gateway is manually changed or ARP caches expire and it discovers the backup router
• B.FHRP is only needed for data center leaf-spine architectures; campus networks can survive router failures without it
• C.Without FHRP, a host automatically discovers the backup router via ICMP Router Discovery (RFC 1256) within 5 seconds
• D.Without FHRP, the STP root bridge takes over as the default gateway, maintaining connectivity through Layer 2