802.1x is the standard that will best help secure the VoIP network from unauthorized users. 802.1x is a protocol that provides port-based network access control, which means that it controls the access to a network port based on the authentication and authorization of the device or the user that is trying to connect to the port. 802.1x can be used to secure the VoIP network by preventing unauthorized devices or users from accessing the network ports that are used for VoIP communication, such as the ports on the switches, routers, or phones. 802.1x can also prevent unauthorized devices or users from accessing the network resources or services that are available through the VoIP network, such as voice mail, call forwarding, or conferencing. 802.1x can also prevent unauthorized devices or users from eavesdropping, intercepting, or modifying the VoIP traffic, by encrypting the communication between the devices and the network ports. The other options are not the standards that will best help secure the VoIP network from unauthorized users, as they either do not provide port-based network access control, do not apply to VoIP communication, or do not exist. References: CISSP - Certified Information Systems Security Professional, Domain 4. Communication and Network Security, 4.2 Secure network components, 4.2.2 Implement secure communication channels, 4.2.2.2 Network access control (NAC); CISSP Exam Outline, Domain 4. Communication and Network Security, 4.2 Secure network components, 4.2.2 Implement secure communication channels, 4.2.2.2 Network access control (NAC)