Which best describes a common denial-of-service attack?

A common denial-of-service (DoS) attack is characterized by overwhelming a server with an excessive amount of traffic or requests, resulting in degraded performance or total unavailability of the services that the server provides. This attack exploits the resource limitations of the server, causing legitimate users to experience problems accessing the services or being completely blocked from reaching them. The goal is typically to disrupt the services provided by the server, rendering it unable to perform its necessary functions.

The other options describe different types of attacks that don't specifically align with the core concept of a denial-of-service attack. For instance, encrypting user files is typical of ransomware, which seeks to extort users by holding their data hostage. Manipulating traffic can pertain to various other network attacks but does not necessarily lead to denial of service; it could refer to traffic analysis or redirection. Lastly, stealing user information is indicative of data breaches or theft attacks, which focus on compromising data integrity or confidentiality rather than service availability. Thus, the defining feature of a denial-of-service attack is its systematic overload of a server, making the description of this attack very precise and targeted.