Which type of network attack disrupts service to legitimate users?

A Distributed Denial of Service (DDoS) attack is designed to make a network service unavailable to its intended users, effectively disrupting access for legitimate users. In a DDoS attack, multiple compromised computer systems are used to flood a target server, application, or network with a barrage of traffic. This overwhelms the resources, making it difficult or impossible for legitimate users to access the services being attacked, such as websites or applications.

The other options describe different types of security threats that do not primarily focus on service disruption for legitimate users. Social engineering involves manipulating individuals into divulging confidential information but does not necessarily disrupt service. Sniffing is a method used to intercept and analyze network packets, which tends to focus on data theft rather than service availability. Session hijacking takes control of a user's session but does not inherently disrupt service; it mainly compromises user credentials and privacy.