With default settings in Windows 7 64-bit which of the following groups is allowed to install unsigned drivers while giving the LEAST amount of permissions?

Authenticated Users is essentially equivalent to Standard Users for driver installation purposes and lacks the elevated rights needed to install unsigned drivers on Windows 7 64-bit.

Windows 7 64-bit enforces kernel-mode code signing by default, requiring all drivers to be digitally signed. The Administrator group is the lowest privilege level that can override this enforcement - for example, by booting with 'Disable Driver Signature Enforcement' or by using test mode - and still install unsigned drivers. Standard Users and Authenticated Users cannot install any drivers regardless of signing status.

The Power Users group was effectively deprecated starting with Windows Vista and Windows 7, and no longer provides elevated driver installation capabilities above a Standard User.

Standard Users do not have the permissions required to install drivers of any kind, signed or unsigned, on Windows 7.