What term describes assurance that a product or service will meet its agreed requirements?

An Underpinning Contract is an agreement between a service provider and a third-party supplier that supports delivery of an IT service, not the customer-facing assurance of agreed requirements.

In ITIL, Warranty describes the fitness-for-use attributes of a service - such as availability, capacity, security, and continuity - rather than the formal contractual document capturing agreed requirements.

A Service Level Agreement formalizes the agreed requirements and committed service levels between a service provider and a customer, giving both parties a documented baseline against which performance is measured and assurance of delivery is given. It is the primary mechanism through which a provider commits to meeting specific service standards.

Utility describes the functionality a service provides to meet a specific need (fitness for purpose), not the formal assurance or documentation of agreed performance requirements.