**The Common Ion Effect**

The common ion effect is a phenomenon observed when a solution containing a common ion is added to another solution containing the same ion. It results in a decrease in the solubility of a slightly soluble salt or a decrease in the ionization of a weak electrolyte. The common ion effect can be explained by Le Chatelier's principle, which states that a system at equilibrium will shift its equilibrium position in response to a stress.

**Sets of Solutions**

A) BaCl2 and Ba(NO3)2:
- BaCl2 and Ba(NO3)2 both contain the common ion "Ba2+." When these salts are dissolved in water, they dissociate into their respective ions, Ba2+ and Cl- or NO3-. The addition of BaCl2 to Ba(NO3)2 would result in an increase in the concentration of Ba2+ ions, leading to the common ion effect. This would decrease the solubility of any other compound containing the Ba2+ ion.

B) NaCl and HCl:
- NaCl and HCl do not contain a common ion. NaCl dissociates into Na+ and Cl- ions, while HCl dissociates into H+ and Cl- ions. The addition of NaCl to HCl would not result in the common ion effect because the ions present in NaCl are different from those in HCl.

C) NH4OH and NH4Cl:
- NH4OH and NH4Cl both contain the common ion "NH4+." NH4OH is a weak base that partially dissociates into NH4+ and OH- ions. NH4Cl is a strong electrolyte that completely dissociates into NH4+ and Cl- ions. The addition of NH4OH to NH4Cl would result in an increase in the concentration of NH4+ ions, leading to the common ion effect. This would decrease the ionization of NH4OH and shift the equilibrium towards the molecular form.

D) None:
- None of the given options satisfy the condition for the common ion effect, except option C (NH4OH and NH4Cl). Therefore, the correct answer is D) none.

In conclusion, the common ion effect is shown by the set of solutions NH4OH and NH4Cl (option C). The presence of a common ion in the solutions causes a decrease in the solubility of a slightly soluble salt or a decrease in the ionization of a weak electrolyte.

Common Ion Effect is the reduction in the ionisation of a weak acid/base due to the presence of a common ion in the solution. If a salt of a weak acid is added to a solution of the acid itself, the dissociation of the acid is diminished further.NH4Cl is a strong electrolyte & so ionise completely while NH4OH being a weak electrolyte does not.so common ion effect is shown by option C