- x.509 v3
The correct answer is 4.
IEEE 802.1x is a standard for NAC (network access control) based on port management. (note: port is being used loosely here to mean connection pathway, not necessarily a physical port or even a Transport layer port.). 802.1x could be described as an authentication proxy service as it allows existing authentication services to be leveraged on specific devices, such as switches, firewalls, proxies, wireless access points, VPN gateways, etc.