1. Go to Run and type gpedit.msc and press Enter.
2. In the Local Group Policy console thus appears find this policy under User Configuration.
3. Set it as Disabled.
4. Your Task Manager option will be restored.
5. If however it is not restored then go to Run again and type gpupdate /force and press Enter.
6. Even then if you don't see the option restored try restarting your computer.