Which type of attack floods a network or server with excessive traffic to make it unavailable to legitimate users?

A server or network can only handle a limited number of requests at once. An attack that deliberately overwhelms that capacity by sending far more traffic or connection requests than the system can process is designed to make the service unavailable to the genuine users it is meant to serve. This is the defining feature of a denial-of-service attack: the goal is disruption of availability, not theft of data.

This is different from attacks that aim to steal or manipulate information. A man-in-the-middle attack secretly intercepts communication between two parties to read or alter it. A phishing attack tricks a user into revealing personal information, usually through a fake message or website. An SQL injection attack inserts malicious code into a database query to access or corrupt stored data. None of these three works by flooding a system with excessive traffic; they rely on deception or exploiting how data is processed, not on overwhelming capacity.

Examination reminder: when a question describes an attack that floods a system to stop legitimate access, look for "denial of service" language such as "unavailable," "overwhelmed," or "flooded."