Q: How can I get my serial number while I'm rebooting?

I'm trying to reboot my MacBook Pro because it had many problems and things i didn't need but when I try to install OS X Mavericks with my apple id (which is the final step into rebooting your mac) it says "OS X Mavericks has not been purchased" when it has been purchased because I used it on my Mac before I decided to reboot.

Besides that I'm trying to chat with the apple support group to fix the problem becuase I can't finish rebooting but i need my serial number but I can't get it while rebooting. The only option I have when clicking the apple icon is

Startup Disk

Restart

Shut down

Can anyone explain how I can my seial number?