Which function of nos involves switching the cpu time between different programs? directory services multitasking file sharing security services

Multitasking  
NOS stands for Network Operating System. So let's take a look at the available options and see what would switch the CPU between different programs.  
Directory services 
* This is an important component of a NOS that deals with managing resources such as users, folders, files, etc. But it's not focused on sharing the CPU between different programs. So it's a bad choice.  
Multitasking 
* This is pretty what it says on the label. It's allowing multiple tasks (programs) to operate on the CPU as if each task has it's own CPU. It effectively gives each program a virtual copy of the CPU with some well known interfaces while preventing the program from directly manipulating the hardware. And this is the correct choice.  
File sharing 
* This is a useful ability of a NOS in that it coordinates and permits multiple programs to share common files. But it's not the correct choice for this question.  
Security services 
* This is the management of user passwords, and protecting different resources such as only permitting users is specific groups from access specific files. Useful, but not the right answer.