Which One of the Following Is the BEST Description of a Service-Based

Which one of the following is the BEST description of a service-based service level agreement (SLA) ?

A) An agreement with an individual customer group, covering all the services that they use
B) An agreement that covers one service for a single customer
C) An agreement that covers service specific issues in a multi-level SLA structure
D) An agreement that covers one service for all customers of that service