What is considered a huge network of compromised systems used by attackers for denial-of-service (DoS) attacks?

A botnet is a network of compromised systems, often referred to as "zombie" computers, that attackers control to carry out various malicious activities, including denial-of-service (DoS) attacks. In a DoS attack, the botnet can flood a target system with excessive traffic, overwhelming its resources and rendering it unavailable to legitimate users. The strength of a botnet lies in its ability to harness the collective computing power of many infected machines, allowing for large-scale and devastating attacks.

Other choices, while related to cybersecurity, do not pertain to the specific concept of a network of compromised systems used for such attacks. Ransomware refers to malware that encrypts files on a victim's system and demands payment for the decryption key. A crypter is a tool used to obfuscate malware to evade detection by security software. Spyware is software designed to secretly monitor and collect information from a user’s system without their consent. Each of these plays a different role in the cyber threat landscape but does not specifically describe a network utilized for conducting denial-of-service attacks.