Arrange the elements N, P, O, S in the order of increasing IE

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Strategy:
 First ionization enthalpy () generally increases across a period and decreases down a group. Account for subshell stability (half-filled ) exceptions.

Step 1: Compare Period 2 vs Period 3
Period 2 () has higher  than Period 3 () due to smaller size.

Step 2: Evaluate  anomalies
- Inside Period 2: Nitrogen () is half-filled (stable), while Oxygen () has pairing repulsion. Thus, .
- Inside Period 3: Phosphorus () is half-filled, while Sulfur () has pairing repulsion. Thus, .

Step 3: C

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