Which of the following is an example of a Denial-of-Service attack?

A Denial-of-Service (DoS) attack aims to make a network service unavailable to its intended users by overwhelming or disrupting the service. The correct answer includes multiple methods that illustrate different facets of DoS attacks.

Flooding the network with illegitimate traffic is a classic example of a DoS attack, as it overwhelms the network resources, making it hard or impossible for legitimate users to access the service. This is often done using tools that generate massive amounts of traffic directed toward a target, consuming bandwidth and processing capacity.

Crashing the operating system with malformed requests represents another form of DoS attack. By sending specially crafted data packets that the system cannot process correctly, the attacker can cause a crash or system failure, rendering the service inaccessible.

Increasing server load with many requests is also a technique used in DoS attacks, where an attacker bombards the server with a high volume of requests, exhausting the server's resources and causing legitimate user requests to be dropped or delayed.

All these methods serve the same purpose—to deny service to legitimate users—thus making it accurate to say that all these choices are examples of Denial-of-Service attacks.