To the weak base, ​NH4OH, if ​ NH4Cl is added which provides the common NH4^+ ions, dissociation of ​NH4OH is suppressed.

For common effect : 1 should be weak and other should be strong and a common ion should be present So ans is C

Common Ion Effect

The common ion effect refers to the phenomenon in which the presence of a common ion reduces the solubility of a compound, causing a shift in the equilibrium towards the formation of a precipitate. This effect occurs when two or more substances containing a common ion are present in a solution.

Explanation of the Options:

A) BaCl2 and Ba(NO3)2
- BaCl2 and Ba(NO3)2 contain the common ion Ba2+. However, the presence of Ba2+ does not affect the solubility of either compound, as they are both highly soluble in water. Therefore, the common ion effect is not observed in this pair.

B) NaCl and HCl
- NaCl and HCl do not contain a common ion. Although they both produce chloride ions (Cl-) in solution, the concentration of chloride ions is not affected by their presence. Therefore, the common ion effect is not observed in this pair.

C) NH4OH and NH4Cl
- NH4OH and NH4Cl contain the common ion NH4+. When NH4OH is dissolved in water, it forms NH4+ and OH- ions. Similarly, NH4Cl also dissociates to form NH4+ and Cl- ions. The presence of NH4+ from both compounds creates a common ion effect, leading to a decrease in the solubility of NH4OH and the formation of a precipitate.

D) NaCN and KCN
- NaCN and KCN do not contain a common ion. Although they both produce cyanide ions (CN-) in solution, the concentration of cyanide ions is not affected by their presence. Therefore, the common ion effect is not observed in this pair.

Conclusion:
The pair that shows the common ion effect is C) NH4OH and NH4Cl, as they contain the common ion NH4+. The presence of NH4+ reduces the solubility of NH4OH, resulting in the formation of a precipitate.