Which pair will show a common ion effect?
a.) $BaC{{l}_{2}}+Ba{{(N{{O}_{3}})}_{2}}$
b.) $NaCl+HCl$
c.) $N{{H}_{4}}OH+N{{H}_{4}}Cl$
d.) $AgCN+KCN$

Hint: To answer this question, we should know about the common ion effect. We should know that common ion refers to the common ion dissolved in the solution. Think about Le Chateliar’s principle and the chemical equilibrium and then see which pair has common ions and which will give a common ion effect among them to get the answer.


Complete step by step answer:
We should first know about common ions. Common ion refers to the common in solution. When we dissolve NaCl and KCl in the same solution, the chlorine ions are common to both salts. We should know that when the common ion effect comes into play in solution, it will be visible as decreasing in solubility of solids. We should know that, when we add common ions the solubility of a compound generally decreases due to a shift in equilibrium.
We should know that, whenever, a solution of an ionic substance comes into contact with another ionic compound with a common ion, the solubility of the ionic substance decreases significantly.
For example: Let us take the example of dissolving NaCl in a solution where the chloride ion ($C{{l}^{}})$ is already present. We should note that the amount of NaCl that could dissolve to reach the saturation point would be lowered. This phenomenon is the common ion effect and plays important roles in pharmaceutical and environmental areas.
Now, we know about the common ion effect. Let us take the above question. We should understand one thing that those ions will show a common ion effect in which one or both of them does not dissociate completely.
If we take option A, of NaCl and HCl. We dissolve both of them to make a solution. We will find that NaCl and HCl dissociate completely as one of them is acid and the other is salt of strong acid and strong base.
In option D, of AgCN and KCN, we will find the same thing that AgCN and KCN dissociate completely as one of them is acid and other is salt of strong acid and strong base.
When we take option C, we will find that $N{{H}_{4}}OH$ is weak base to which $N{{H}_{4}}Cl$ is added which provides common ion $NH_{_{4}}^{+}$ and dissociation of $N{{H}_{4}}OH$ is suppressed. Or we can say that $N{{H}_{4}}Cl$ is a strong electrolyte & ionise completely. But in the case of $N{{H}_{4}}OH$, it is being a weak electrolyte so it does not ionise. $N{{H}_{4}}Cl$ provides the common ion $NH_{_{4}}^{+}$ which further suppresses the ionisation of $N{{H}_{4}}OH$ so, as to decrease the $O{{H}^{-}}$ ion concentration so that higher group cations will not get precipitated.
So, the correct answer is “Option C”.

Note: There are some applications of common ion effect that we use in solubility applications. We should note that sodium carbonate is added to the raw water to reduce the hardness of the water. We should note that sodium carbonate is highly soluble in water, it is added to water, to precipitate out sparingly soluble calcium carbonate.$C{{l}^{}})$
We also use the common ion effect in manufacturing of soaps. When we add sodium chloride, it reduces the solubility of the soap salts. The soaps precipitate due to a combination of common-ion effect and increased ionic strength.