doorsill, doorstep, threshold - the sill of a door; a horizontal piece of wood or stone that forms the bottom of a doorway and offers support when passing through a doorway.
The configuration information in the RADIUS server defines what will be installed on the NAS. The figure below illustrates the RADIUS authentication and authorization sequence. Accounting. The accounting features of the RADIUS protocol can be used independently of RADIUS authentication or authorization. The RADIUS accounting functions allow data to be sent at the start and end of sessions, indicating the amount of resources (such as time, packets, bytes, and so on) used during the session.
I think the correct answer from the choices listed above is the third option. The purpose of a parenthetical citation is to refer the reader to the source information on the Works Cited page. It would make it easier for the reader to locate the complete citation. Hope this answers the question.