In ITIL 4, the service level management practice is responsible for setting clear, agreed targets for service performance and ensuring that services are delivered in line with these targets.
A key tool used in this practice is the service level agreement (SLA). An SLA is an agreement between a service provider and a customer that:
Defines the expected level of service
Documents specific, measurable service targets (such as availability, response time, resolution time)
Provides a basis to monitor and measure performance against those targets
Because SLAs describe measurable service targets and are used to assess whether the service provider is meeting those targets, they are explicitly used as a tool to help define and measure performance. This aligns directly with the wording in the question.
A. A continual improvement register (CIR)The CIR is used in the continual improvement practice to record, track, and manage improvement opportunities and activities. It supports managing improvements, not defining and measuring service performance targets.
B. An incident recordThis belongs to the incident management practice. Incident records are used to log, manage, and track incidents (unplanned interruptions or reductions in service). They are not designed as tools to define or measure ongoing service performance.
C. A change schedulePart of the change enablement practice, the change schedule is used to plan and communicate the timing of changes. It coordinates when changes will occur but does not define or measure service performance levels.
Therefore, the correct answer is D. A service level agreement, as it is the formal tool used to define and measure service performance in ITIL 4.
ITIL 4 Foundation: Service level management practice – purpose and key terms (including service level agreements)
ITIL 4 Foundation: Concept of SLAs as documents that define and measure service performance targets
ITIL 4 Foundation: Overviews of Continual Improvement, Incident Management, and Change Enablement practices and their key records (CIR, incident records, change schedules)