Which of the following is a common use of the RADIUS protocol?

The RADIUS (Remote Authentication Dial-In User Service) protocol is primarily used for user authentication, which makes it the correct choice in this context. RADIUS facilitates the process of verifying the identity of users or devices before granting access to a network or specific resources within that network. This authentication is especially important in environments where secure access is crucial, such as in corporate networks or internet service provider (ISP) environments. RADIUS operates by transmitting user credentials to a centralized server that verifies the information against a database, thus ensuring that only authorized users can access the system.

The other options do not align with the primary function of RADIUS. File transfer involves moving files across networks, which RADIUS does not address. Data encryption refers to securing data by converting it into a coded format, a task outside the scope of RADIUS. Network topology mapping deals with visualizing and representing the setup of a network, which is not related to the authentication processes provided by RADIUS. Thus, user authentication remains the definitive application of the RADIUS protocol.