Heterogenous catalyst is the type of catalyst, whose physical state (solid, liquid, aqueous, gas) is not same as reactants and products.

For reaction (a), catalyst NO(g) is in the same physical state or phase with the reactants and products which are in gas phase. So (a) here heterogeneous catalysis is not involved. So (iii) is wrong.

For (d), the catalyst HCl(l) is in same phase with all the reactants and products which are in either liquid or aqueous phases. So this is also not heterogeneous catalysis.

Therefore, option (ii) and (iv) are also wrong.

In (b), catalyst Pt(s) is an heterogeneous catalyst, because it is not in same phase with the reactant SO₂(g) and the product SO₃(g).

Similarly in (c), catalyst Fe(s) is an heterogeneous catalyst, because it is not in same phase with the reactant N₂(g), H₂(g) and the product NH₃(g).

So both (b) and (c) are the examples of heterogeneous catalysed reactions.

Therefore option (i) is the correct.