Which EEM applet syntax registers an applet that can be executed on demand (manual invocation)?

The correct answer registers an EEM applet for on-demand execution using event none. This keyword explicitly indicates that the applet does not have an associated event trigger and must be invoked manually. The other options are incorrect because they either omit the necessary event keyword for manual invocation (event manager applet ondemand action...), use an invalid event keyword (event register or event manual), or are syntactically incomplete for defining an event-less applet. The event none keyword is the standard and correct way to define an EEM applet that can be run whenever needed by an administrator.