The correct order of increasing valency among Al, Cs, Ba and K is

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Al< Cs < Ba < K

K< Ba < Cs < Al

Cs < K = Ba < Al

K=Cs <Ba< Al

answer is D.

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Explanation:

Cesium (Cs) belongs to Group 1A (alkali metals) of the periodic table and has a valence electron configuration of $6 s^{1}$ . It has a valency of +1 by losing its single $6 s^{1}$ electron.
Barium (Ba) belongs to Group 2A (alkaline earth metals) of the periodic table and has a valence electron configuration of $6 s^{2}$ . It has a valency of +2 by losing its two $6 s^{2}$ electrons.
Potassium (K) belongs to Group 1A (alkali metals) of the periodic table and has a valence electron configuration of $4 s^{1}$ . It has a valency of +1 by losing its single $4 s^{1}$ electron.
Aluminum (Al) belongs to Group 3A of the periodic table and has a valence electron configuration of $3 s^{2} 3 p^{1}$ . It has a valency of +3 by losing its three $3 s^{2} 3 p^{1}$ electrons.

Therefore the correct order is K=Cs <Ba< Al.

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