Final answer:
Option C, NH₄OH + NH₄Cl, will show the common ion effect.
Step-by-step explanation:
The pair that will show the common ion effect is option C. NH₄OH + NH₄Cl.
The common ion effect occurs when two substances, one a weak base and the other a salt containing the conjugate acid of the base, are dissolved in the same solution.
In this case, NH₄OH is a weak base and NH₄Cl is a salt. When both substances are dissolved in the same solution, NH₄OH will ionize to produce NH₄⁺ and OH⁻ ions, while NH₄Cl will dissociate to produce NH₄⁺ and Cl⁻ ions.
Since both NH₄⁺ ions come from different sources, the concentration of NH₄⁺ is higher than it would be if NH₄Cl was not present. This is an example of the common ion effect.