System hardening involves disabling unnecessary services and uninstalling unnecessary software from the system. System hardening also involves disabling unused accounts and patching the system. The rest of the answers are incorrect because they are all network hardening techniques and not system hardening techniques. MAC filtering controls which systems can send data to other
systems, port security controls which systems can connect to a port by MAC address, and 802.1x controls who has access to a wired or wireless network by using a central authentication server.