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

This describes a generic contract clause rather than an SLA, which is a standalone agreement focused on documented service levels and targets, not general contractual responsibilities.

An agreement between a service provider and an internal organization describes an Operational Level Agreement (OLA), a different ITIL construct that supports SLA delivery.

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

In ITIL, an SLA is the agreement between the IT service provider and the customer - whether internal or external - that documents service level targets and the responsibilities of both parties in meeting those targets. This definition is distinct from an Operational Level Agreement (OLA), which is between internal teams, and an Underpinning Contract (UC), which is with an external supplier. Choosing 'customer' as the counterparty is what makes this definition correct and complete.