What class of malware hides the existence of other malware?

The correct choice is a rootkit, which is designed specifically to conceal the presence of certain objects or activities within a system. Rootkits operate at a low level, often interfacing with the operating system directly to hide the existence of other malicious software, making detection by traditional anti-malware tools difficult. They can disguise their own files, processes, and system settings, thus allowing various types of malware, including viruses or Trojans, to operate undetected.

In contrast, viruses, Trojans, and worms serve different primary functions. Viruses attach themselves to legitimate programs or files to replicate and spread, while Trojans masquerade as legitimate software to trick users into executing them. Worms are standalone malware that self-replicate to spread across networks. None of these classes inherently focus on hiding other malware, which is the distinguishing feature of a rootkit. This unique capability of rootkits makes them a significant threat in cybersecurity, as they maintain stealthy control over a compromised system, often allowing other malicious activities to go unnoticed.