The BEST description of an incident is:

The existence or absence of a backup service is irrelevant to the definition of an incident; the definition is based on the unplanned nature of the interruption, not redundancy status.

The formal ITIL definition states that an incident is 'an unplanned interruption to an IT service or a reduction in the quality of an IT service.' This captures both total outages and partial degradation scenarios, and the word 'unplanned' explicitly separates incidents from scheduled maintenance or change activity.

Planned disruptions such as approved maintenance windows are not incidents; the definition requires the interruption to be unplanned, which this option omits.

An incident is defined by its characteristics (unplanned interruption or quality reduction), not by the channel through which it is reported; unreported incidents still qualify as incidents.