The correct answer is Set requiredDuringSchedulingIgnoredDuringExecution under spec.affinity.nodeAffinity in the Pod manifest.

Using requiredDuringSchedulingIgnoredDuringExecution under spec.affinity.nodeAffinity creates a hard node affinity constraint that the scheduler must satisfy. When you define a selector that matches nodes labeled tier=web the scheduler will only place the Pod on nodes that match. This enforces that the Pod is scheduled only onto tier=web nodes.

Use preferredDuringSchedulingIgnoredDuringExecution under spec.affinity.nodeAffinity with a weight and a match for tier=web is incorrect because it is a soft preference. The scheduler can place the Pod on other nodes if no matching nodes are available.

Taint the target nodes and add a matching toleration to the Pod to place it on tier=web nodes is incorrect because taints and tolerations are a different mechanism that admits or repels Pods from nodes rather than expressing a node affinity rule.

Add a nodeSelector entry in the Pod spec with tier=web is incorrect in this context because the question asked specifically about writing a node affinity rule. NodeSelector would also restrict placement but it is not the node affinity construct the question requests.

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