An example of a DDoS attack is primarily associated with which action?

A Distributed Denial of Service (DDoS) attack specifically aims to make a service unavailable by overwhelming the targeted server or network with an excessive amount of traffic. This is typically achieved by using multiple compromised computer systems, which send a flood of requests simultaneously. The intention is to exhaust the server's resources, such as bandwidth, memory, and processing power, leading to legitimate users being unable to access the targeted service.

The other options pertain to different types of cybersecurity threats or actions. For example, encrypting sensitive data relates to protecting data confidentiality rather than disrupting service, while stealing user credentials involves gaining unauthorized access rather than overwhelming a server. Blocking unauthorized access is a defensive measure and does not directly relate to the DDoS attack mechanism, which is inherently about creating disruption through excessive traffic rather than preventing attacks. Thus, the correct choice illustrates the core function of a DDoS attack effectively.