What is a key characteristic of a non-transparent proxy?

A non-transparent proxy is primarily characterized by its requirement for client software configuration. This means that, unlike transparent proxies, which operate without the need for any changes on the user's device, non-transparent proxies require users to manually configure their applications or systems to direct traffic through the proxy server.

This configuration is essential as it ensures that the traffic is explicitly routed through the proxy, allowing it to perform its functions, such as filtering, logging, and potentially altering content. By needing client-side configuration, non-transparent proxies offer users a degree of control and can enforce specific policies or access permissions based on user needs.

The other options represent characteristics that do not define a non-transparent proxy in the same way. While some non-transparent proxies can be invisible to the end user, that does not define their key characteristic, as the visibility can depend on how the proxy is implemented. Hiding user IP addresses is a common function of many proxies but again does not uniquely identify non-transparent proxies. Lastly, while optimizing content for the client can occur, it's not inherent to the non-transparent proxy's definition; rather, it's a feature that might be present in various types of proxies.