The isoelectronic series is correctly arranged in order of increasing radius is option D. Ca2+ < K+ < Ar < Cl.
Since k+, Ca2+, Cl- and Ar represent the same kind of e- (18) due to this the positive charge increased. The size should be decreased due to the net attraction on outer e- increased and a negative charge should increase the size because the electronic repulsion should be increased.
Therefore, the option d is correct.
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