Which of the following BEST defines a Service Level Indicator (SLI)?

Comprehensive and Detailed Explanation From Exact Extract:

Google’s definition is explicit: “An SLI is a carefully defined QUANTITATIVE measure of some aspect of the level of service provided.” (SRE Book – Chapter: Service Level Objectives). Examples include error rate, latency, throughput, and availability. SLIs are measurements, not targets—targets are SLOs.

Option D repeats Google’s definition almost exactly.

Option C incorrectly describes an SLO (a target), not an SLI.

Options A and B mention subjective assessments—SRE explicitly rejects subjectivity in measurement, stating: “SLIs must be objective and measurable.”

Thus, D is the correct and SRE-authentic answer.