What is a rootkit used for?

A rootkit is a type of software that is specifically designed to gain unauthorized access to a computer system and remain hidden while doing so. It allows a malicious actor to conceal their presence and the activities of other harmful software that may be running on the infected system. By using a rootkit, attackers can manipulate the system without detection, making it easier for them to execute other malicious actions, such as stealing data, controlling the system remotely, or installing additional malware.

While the other options describe various functions that software might perform, they do not accurately capture the primary purpose of a rootkit. For instance, rootkits are not typically used to spread viruses (which is more the function of a virus itself) or to enhance system performance. The ability to hide the operations of other malicious software is what fundamentally characterizes rootkits, making this the correct answer.