Which option is a characteristic of the RADIUS protocol?

A.
uses TCP

B.
offers multiprotocol support

C.
combines authentication and authorization in one process

D.
supports bi-directional challenge

Explanation:
http://www.cisco.com/en/US/tech/tk59/technologies_tech_note09186a0080094e99.shtml
Authentication and Authorization
RADIUS combines authentication and authorization. The access-accept packets sent by the RADIUS
server to the client contain authorization information. This makes it difficult to decouple
authentication and authorization.
TACACS+ uses the AAA architecture, which separates AAA. This allows separate authentication
solutions that can still use TACACS+ for authorization and accounting. For example, with TACACS+, it
is possible to use Kerberos authentication and TACACS+ authorization and accounting. After a NAS
authenticates on a Kerberos server, it requests authorization information from a TACACS+ server
without having to re-authenticate. The
NAS informs the TACACS+ server that it has successfully authenticated on a Kerberos server, and the
server then provides authorization information.
During a session, if additional authorization checking is needed, the access server checks with a
TACACS+ server to determine if the user is granted permission to use a particular command. This
provides greater control over the commands that can be executed on the access server while
decoupling from the authentication mechanism.