Which of the following pairs will show common ion effect?

Solution: Common ion effect is show when weak electrolyte and strong electrolyte is mixed. $N{H}_{4}OH$  is weak base to which $N{H}_{4}Cl$  is added which provides common $N{H}_4$  ion dissociation of  $N{H}_{4}OH$  is suppressed. Since  $N{H}_{4}OH$  is a weak electrolyte whereas $N{H}_{4}Cl$  is a strong electrolyte it shows the common ion effect.