A Service Level Agreement is best described as a documented agreement between a service provider and a customer that identifies the services and their agreed levels, so option B is correct. In ITIL, the SLA is a formal mechanism for creating shared understanding of expected and achieved service quality. It may include metrics related to utility, warranty, sustainability, and user experience, depending on context. It is more than an informal discussion and is not just an internal guideline. It also is not limited to vendor penalty clauses, although contractual elements may exist around it. The purpose of an SLA is to clarify what will be provided, how performance will be measured, and what both parties can reasonably expect. This helps support transparency, accountability, and effective service relationships.