Final answer:
The sign-in method that enables multiple users to run applications simultaneously on the same computer is fast user switching.
Step-by-step explanation:
The sign-in method that allows multiple users to have applications running on the computer at the same time is C. fast user switching. Fast user switching is a feature in some operating systems that allows users to sign into a computer while other users are still logged in and their applications are running in the background. This feature is quite useful in scenarios where a computer is shared among several users and there is a need to switch between users without closing applications or logging off completely.