Which one of the following is the BEST definition of reliability?

Describing reliability as the availability of a service is incorrect - availability measures the proportion of agreed service time that a service is actually accessible, which is a related but distinct concept.

The level of risk affecting a service describes risk exposure or risk posture, not the operational characteristic of reliability.

Reliability is formally defined in ITIL as a measure of how long a service or configuration item can perform its agreed function without failing, which directly corresponds to the metric Mean Time Between Failures (MTBF). This definition is distinct from availability, which measures the percentage of agreed time a service is accessible, and from recoverability, which measures restoration speed after a failure.

How quickly a service can be restored describes recoverability or maintainability, measured as Mean Time to Restore (MTTR), not reliability.