An analytics Pod must run only on nodes labeled disk=ssd. Other nodes have sufficient resources but no such label. Which rule expresses a mandatory placement constraint?

1. An annotation with the text disk=ssd
2. requiredDuringSchedulingIgnoredDuringExecution node affinity matching disk=ssd
3. A Pod label disk=ssd without node affinity or a selector
4. preferredDuringSchedulingIgnoredDuringExecution node affinity matching disk=ssd