None of those, get out of system preferences, the startup disk is as its name suggests the disk your Mac starts up from.
I'm assuming that you don't have a Hard Disk icon on your desktop as this is (to me) the easiest way in.
Select finder in your dock, look down the left side column and under devices you should see Mackintosh HD
If you don't, with finder in the menu bar select finder and then preferences, this should open a new Finder Preferences window. In the new window select the sidebar icon, then look down the list showing and put a tick in the box next to your Hard Disk in the Devices section. Close the window.
Select finder in the dock again, now your hard disk should be showing in the side bar, select it
Select the Library folder
Select the Preferences folder
Select the SystemConfiguration folder
Drag network interfaces.plist file to the trash
Reboot