The value of the constant A in the Debye - Huckel-Onsager equation, λm=λmo−A√C depends on temperature, charges of the ions, dielectric constant of the solvent and also the viscosity of the solvent. Here, since we are not given the solvent, we will assume that the solvent is same for each case. Here, the deciding factor are the charges on the ions. Again, in NH4Cl and NaBr the charges on the ions are same. Hence the pair of electrolytes that will possess same value for the constant (A) will be NH4Cl,NaBr.