I do not wish to authorize any extensions at this time.
I think you may have already gone past it (or might be the next screen ??). Are you trying to authorize extensions or the core software? Did you request your authorizations from myesri? If so, you can chose the third option and point to the file that you downloaded or were emailed. If you already have core installed, and are trying it authorize extensions, use the middle option and you will get the page to enter authorization code for the extension.
But it sounds like this is your first install, so you will want to select option 1 or 3 ....or you can cancel out of this part, get your authorization email/file and then start the authorization process again.
Here is a help document that might help
Authorizing your software—Help | ArcGIS for Desktop