Which command globally enables AAA on a device?

Which command globally enables AAA on a device?
A . aaa new-model
B . aaa authentication
C . aaa authorization
D . aaa accounting

Answer: A

Explanation:

To configure AAA authentication, enable AAA by using the aaa new-model global configuration command. AAA features are not available for use until you enable AAA globally by issuing the aaa new-model command.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/security/configuration/guide/fsecur_c/scfathen.ht ml