The correct answer is C. In Cisco UCS Manager AAA configuration, external authentication is built hierarchically. First, individual RADIUS servers are defined. Those servers are then placed into a provider group, and that provider group is referenced by an authentication domain. When a user logs in, UCS Manager consults the configured authentication domain and then tries the RADIUS servers that belong to the associated provider group. To use all four RADIUS servers for authentication, the proper design is to place the four servers into one provider group and then attach that provider group to one authentication domain.
This structure allows Cisco UCS Manager to try the RADIUS servers in the provider group according to AAA behavior before falling back or failing authentication. The other answers reverse the hierarchy or misuse the object relationships. Authentication domains do not contain multiple independent provider groups in the way described by options A and B, and RADIUS servers are not placed directly under an authentication domain as suggested in D.
Therefore, the correct configuration is four RADIUS servers in one provider group, with that single provider group assigned to one authentication domain.