Which authentication methods should a network administrator combine to validate a client's access to the network?

Combining IEEE 802.1X with EAP (Extensible Authentication Protocol) is an effective method for validating a client's access to a network. IEEE 802.1X is a network access control protocol that provides an authentication mechanism for devices wishing to connect to a LAN or WLAN. It works by using a client-server model where the client (the device seeking access) must authenticate to an authentication server, typically using a RADIUS server.

EAP enhances this process by allowing various authentication methods such as certificates or usernames and passwords to be used within the framework of IEEE 802.1X. By utilizing both of these protocols together, a network administrator ensures that only authorized users can gain access to the network. This layered approach significantly enhances security by requiring both identification and verification of users before granting access, helping to prevent unauthorized access.

The other options, while they may provide certain security measures, do not form as robust a combination for authentication as the first choice does in terms of layered security and comprehensive access control.