The CORRECT order of first ionisation enthalpy is:

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Strategy:
 Order Period 3 elements (Al, Mg, S, P) by ionization enthalpy , accounting for subshell stability anomalies ( vs  and  vs ).

Step 1: Compare Mg and Al
-  () has a fully filled subshell.  () has one electron in a higher-energy -orbital. Thus, .

Step 2: Compare P and S
-  () is half-filled and stable.  () experiences electron pairing repulsion. Thus, .

Step 3: Combine
Across the period, values increase significantly from Grp 2/13 to Grp 15/16.
Order: .

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