A cluster only has one storage pool.

Nutanix documentation on capacity reservation explains that a storage container can reserve a portion of storage so that a minimum amount of capacity remains available for that specific container. When reservations are configured, one container may retain protected usable space while other containers in the same storage pool can run out of usable capacity first, even though the pool still shows free space overall. That exactly matches the scenario in the question, which is why C is the correct answer. ( Nutanix )

This is a subtle storage-administration concept. The question is not saying the pool itself is full; it says all but one container are out of space while the single pool still has free space. That pattern is much more consistent with reservation policy than with hardware failure. Failed nodes or drives would usually surface additional health symptoms and would not neatly explain one container still having preserved capacity while the others are exhausted. Nutanix intentionally allows reservation for predictable capacity control, and that is the feature responsible here. Hence A, B, and D are distractors, while C is correct. ( Nutanix )