Common Ion

NH₄OH + NH₄Cl

Those pairs will show common ion effect in which one or both of them do not dissociate completely.

NH₄OH is a weak base and it does not dissociate completely. So there will be common ion effect here

but NaCl , AgCN,KCN as well as BaCl₂ and Ba(NO₃)₂ do not dissociate

none of AgCN and KCN were not a weak acid or a weak base.

oops;
yeah AgCN nor KCN are weak.
they dissociate completely

Those pairs will show common ion effect in which one or both of them do not dissociate completely.

basically this is the concept

Yes the answer is (c) as NH₄OH AND NH₄Cl acts as a weak base as in common ion effect we need to have same ions and weak electrolytes