For which of the following thresholds would it be MOST important to set an alert for thin provisioned storage?

Write cache used is a performance metric that can indicate bottlenecks but does not directly warn about the physical storage pool exhaustion that is critical for thin-provisioned volumes.

Fragmentation percentage impacts performance over time but is not the most critical alert to prevent a complete storage outage due to insufficient physical capacity in a thin-provisioned environment.

IOPS (Input/Output Operations Per Second) is a performance metric that measures throughput but does not directly indicate the physical space availability of the underlying storage pool for thin-provisioned volumes.

For thin provisioned storage, setting an alert on 'Capacity used' is most important because if the physical storage pool backing the thin-provisioned volumes runs out of space, new write operations will fail, leading to critical service interruptions. Monitoring this threshold allows proactive intervention by adding more physical storage before an outage occurs.