Since electron donating gps. decrease the acid strength therefore 4-methoxybenzoic acid is a weaker acid because methoxy sp. is E. D. G. than benzoic acid. Further since electron withdrawing gps. increase the acid strength, therefore both 4 nitrobenzoic acid and 3, 4-dinitrobenzoic acids are stronger acid than benzoic acid. Further due to the presence of additional NO2 gp at m-position wrt COOH gt, 3, 4-dinitrobenzoic acid is a lattice stronger acid than 4-nitrobenzoic acid. Thus the increasing order of acid strength will be : 4-methoxybenzoic acid < benzoic acid < 4-nitrobenzoic acid < 3, 4, dinitrobenzoic acid.