Which action could have prevented a breach in a datacenter where a server was accessed while shut down but still connected to the network?

Disabling Wake-on-LAN in the server BIOS is a proactive measure for preventing unauthorized access to a server, even when it is turned off yet still connected to the network. Wake-on-LAN (WoL) is a feature that allows a network-connected device to be powered on remotely by a network message. If this feature is enabled, an attacker could potentially send a magic packet to wake the server while still being in a powered-down state, allowing them to gain access before proper security measures come into effect.

By disabling Wake-on-LAN, the server can no longer be woken remotely through this method. This limits the possibility of unauthorized access and ensures that the server remains in a non-operational state unless someone physically powers it on. This is especially crucial for securing data centers where sensitive information may reside.

The other options, while beneficial to overall server security, do not address the specific vulnerability of a powered-off server that can be remotely activated. Shutting down unneeded ports helps limit exposure but does not eliminate the risk of WoL; installing anti-malware software protects against malicious software but does not prevent unauthorized physical access; and disabling non-essential services enhances security but does not resolve the specific scenario of remote power activation through WoL.