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

Describing responsibilities of each party is a characteristic shared by many agreement types and does not specifically define an SLA as distinct from a contract or OLA.

An agreement between the service provider and an internal organization describes an Operational Level Agreement (OLA), not an SLA.

An agreement between a service provider and an external supplier describes an Underpinning Contract (UC), not an SLA.

An SLA is an agreement between an IT service provider and a customer that documents agreed service level targets and the mutual responsibilities of both parties. ITIL distinguishes SLAs from Operational Level Agreements (OLAs, which are internal) and Underpinning Contracts (UCs, which are with external suppliers), making the customer-facing scope the defining characteristic.