The open standard among the given options for First Hop Redundancy Protocols (FHRPs) is VRRP (Virtual Router Redundancy Protocol).
- TACACS+ is a protocol for network access control and is not a FHRP.
- GLBP (Gateway Load Balancing Protocol) and HSRP (Hot Standby Router Protocol) are proprietary to Cisco.
So, the correct answer is VRRP.