The metric that measures how quickly something is detected/identified is Mean Time to Detect (MTTD).
Although MTTD is often used in incident detection, it directly matches the wording "how quickly ... are identified" because it measures the time between occurrence and detection.
Secbay Press defines MTTD explicitly as the time to detect an issue:
Exact extract (Secbay Press):
"Mean Time to Detect (MTTD) is... the average time taken to identify and detect a security incident... Mean time to detect is how long it took... to when it was detected." Why the other options are not best:
* KPI is a category/type of measure (a key metric), not the specific metric itself. Secbay distinguishes metrics vs KPIs and lists MTTD as a KPI example.
* SLO is a service objective/target, not a measurement of detection speed by itself.
* Alert volume measures quantity of alerts, not detection time.
Exact extract (Secbay Press):
"KPI: Mean Time to Detect (MTTD) and Mean Time to Respond (MTTR)."