What is a DDoS attack?

A Distributed Denial of Service (DDoS) attack is characterized by using a multitude of compromised systems—often referred to as a botnet—to overwhelm a target system with an excessive volume of traffic. The primary goal of a DDoS attack is to render the target's services unavailable to legitimate users by focusing an influx of requests that the target cannot handle due to its limited capacity. This tactic underscores the "distributed" aspect of the attack since the traffic originates from various sources, making it challenging to mitigate effectively.

Other types of attacks mentioned in the choices focus on different methods and targets. For instance, a targeted phishing attack zeroes in on individuals with the intent of tricking them into divulging sensitive information, which is distinctly different from the traffic-overloading strategy of a DDoS attack. Similarly, exploiting vulnerabilities in web applications is an approach aimed at taking advantage of specific software weaknesses, while unauthorized access methods could involve various techniques to bypass security controls but do not necessarily cause disruption through traffic flooding as DDoS attacks do. Thus, the correct option clearly portrays the nature of a DDoS attack, emphasizing its reliance on distributed resources to launch a large-scale assault on the target's availability.