What type of server is a master server that is designed to listen to individual compromised endpoints and respond with appropriate attack commands?

The type of server described as a master server that listens to compromised endpoints and responds with attack commands is indeed a command and control server. These servers play a crucial role in cyber operations, particularly in the context of malware and botnets.

A command and control server is employed by attackers to manage compromised systems, or "bots," in a botnet. When endpoints (such as computers or devices) become infected with malware, they often connect back to the command and control server to receive instructions. This allows the attacker to control the infected devices to perform various malicious activities, such as launching attacks, stealing data, or spreading malware to other devices.

In contrast, the other options do not fit the definition described. A bot refers to the individual compromised endpoint itself, while web servers typically host websites and directories; directory services manage network resources rather than control compromised devices. Thus, the role of a command and control server is uniquely related to managing and directing actions across compromised endpoints in a coordinated fashion.