The correct order of atomic radii is:

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Strategy:
 Atomic radii in the lanthanide series decrease from left to right due to "Lanthanoid Contraction" (poor shielding by  electrons). Compare these transition-like metals with small non-metals from early periods.

Step 1: Apply Lanthanoid Contraction
Atomic numbers: Cerium (), Europium (), Holmium ().
The radius decreases as the atomic number increases within the  series.
Order: .

Step 2: Compare with Nitrogen
Nitrogen () is a small non-metal in Period 2. Its atomic radius () is signific

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