how to factory reset mac mini 2011

I followed the instructions but I am stumped. When I get to "Erase Free Space" tab, it ask me what to choose. Either I want the Fastest, Most Secure or between them both. I did the fastest option and I feel like it didn't work.

In case you ask, my Mac Mini has Mac OS X Lion 10.7.5.

Anyone knows what I can do please help me. I am desperate.

What makes you "feel: the erase did not work?

Also see:

What to do before selling or giving away your Mac

Boot into recovery (press Command and R buttons during boot), format your hard drive using Disk Utility and reinstall OS X Lion