The ionic radii of K, Na, Al and Mg are in the order:

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Strategy:
 Compare elements based on their position in the periodic table. Across a period, radius decreases due to higher . Down a group, radius increases as more shells are added.

Step 1: Group comparison
 and  belong to Group 1. Since  is in a lower period (Period 4) than  (Period 3), .

Step 2: Period comparison
 are isoelectronic (10e) in Period 3.
As charge increases ( increases from  to ), size decreases.
Order: .

Step 3: Combine
.

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