Which protocol allows for the automatic selection and simultaneous use of multiple available gateways as well as automatic failover between those gateways?

Which protocol allows for the automatic selection and simultaneous use of multiple available
gateways as well as automatic failover between those gateways?

A.
IRDP

B.
HSRP

C.
GLBP

D.
VRRP

Explanation:
To provide a virtual router, multiple switches (routers) are assigned to a common GLBP
group. Rather than having just one active router performing forwarding for the virtual router
address, all routers in the group can participate and offer load balancing by forwarding a

portion of the overall traffic. The advantage is that none of the clients have to be pointed
toward a specific gateway address—they can all have the same default gateway set to the
virtual router IP address. The load balancing is provided completely through the use of
virtual router MAC addresses in ARP replies returned to the clients. As a client sends an
ARP request looking for the virtual router address, GLBP sends back an ARP reply with the
virtual MAC address of a selected router in the group. The result is that all clients use the
same gateway address but have differing MAC addresses for it.
Reference:
Configuring GLBP
(http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_glbp.html)