What is the BEST definition of an internal service?

This option incorrectly characterizes an internal service as a 'supporting service' and limits the scope to functions within the IT service provider, which is too narrow and conflates internal services with underpinning supporting services.

A service delivered by an outsourced provider to customers is an external service arrangement, not the definition of an internal service.

A service delivered under a contract to enable a customer to achieve business outcomes describes an external or contractual service relationship, not an internal service between organizational units.

ITIL defines an internal service as one delivered between departments or business units that belong to the same organization, meaning both the provider and consumer are part of the same enterprise. This definition is important because internal services are typically governed by operational level agreements (OLAs) rather than external contracts or SLAs. It distinguishes services exchanged within an organization from external services delivered to outside customers or partners.