What function does a rootkit serve?

A rootkit serves the function of providing unauthorized users with remote control of a computer. This type of malicious software is specifically designed to remain hidden from detection while allowing an attacker to gain elevated privileges and manipulate system settings. Rootkits can operate at a low level within the operating system, making them particularly insidious as they can mask their presence and conceal other types of malware that might be installed.

The stealthy nature of rootkits makes them a powerful tool for cybercriminals, enabling them to maintain continuous access to compromised systems, exfiltrate sensitive data, and execute other malicious activities without the knowledge of legitimate users or security software.

In the context of the other options: providing remote access to authorized users pertains more to legitimate software, enhancing system performance is not a function of a rootkit, and protecting systems from malware is the opposite of what a rootkit does, as it compromises system integrity rather than enhancing it.