Final answer:
The system administrator should implement PEAP (Protected Extensible Authentication Protocol) to require username and password authentication for a wireless network.
Step-by-step explanation:
To configure a wireless network to require username and password authentication, the system administrator should implement Protected Extensible Authentication Protocol (PEAP). PEAP is an authentication method that uses a username and password, and is commonly used in conjunction with a RADIUS server that handles the actual authentication process against a user database. It encapsulates the identity of the user and provides secure authentication mechanisms.
Other options listed like WPS (Wi-Fi Protected Setup) simplifies the process of connecting devices to the network but isn't focused on username/password authentication. TKIP (Temporal Key Integrity Protocol) is an encryption protocol used in Wi-Fi networks. PKI (Public Key Infrastructure) is related to the use of certificates and encryption but not directly used for username/password authentication in Wi-Fi.