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)