There may be many valencies for some items. Valency has no sign; there is neither positive nor negative valency. The Greek word 'valentia,' which means strength and ability, is the source of the term valency. This idea was created to comprehend the molecular makeup of both organic and inorganic molecules.

In a balanced reaction the number of molecules of reactant on the left-hand side should be equal to the number of molecules of product on the right-hand side.

(A) Since all three atoms are balanced on both sides of this reaction, this reaction is the one that must occur.
(B) Although the hydrogen and oxygen ions are present and in balance in this reaction, the sodium ion is not, hence the reaction will not be balanced.
(C) Since only the oxygen atoms in this reaction are balanced, the condition will not be balanced. However, nitrogen and hydrogen are unbalanced.
(D) Only the sodium atoms in this reaction are present in a balanced state on both sides, but oxygen and hydrogen are not present in a balanced state, hence this reaction will be imbalanced.

$2\mathrm{Na}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}+{\mathrm{H}}_2$

In this reaction in both sides of the reaction all the three atoms are balanced, so this reaction will be the correct answer.

Hence option (A) is correct.