To solve this problem, we need to understand the concept of heterogeneous catalysis.$\newline$Heterogeneous catalysis occurs when the catalyst is in a different phase than the reactants.$\newline$Let's analyze each option to determine which one involves heterogeneous catalysis:$\newline$Option 1: Oxidation of sulphur dioxide into sulphur trioxide in the presence of oxides of nitrogen.$\newline$• In this reaction, both the reactants and the catalyst (oxides of nitrogen) are in the gas phase.$\newline$• This is an example of homogeneous catalysis.$\newline$Option 2: Hydrolysis of sugar catalyzed by $H^+$ ions.$\newline$• Here, both the sugar and the $H^+$ ions are in the aqueous phase.$\newline$• This is also an example of homogeneous catalysis.$\newline$Option 3: Decomposition of ozone in presence of nitrogen monoxide.$\newline$• In this reaction, both ozone and nitrogen monoxide are gases.$\newline$• This is another example of homogeneous catalysis.$\newline$Option 4: Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron.$\newline$• In this case, dinitrogen and dihydrogen are gases, while the catalyst (finely divided iron) is a solid.$\newline$• This is an example of heterogeneous catalysis.$\newline$Therefore, the correct option is Option 4, where the catalyst is in a different phase than the reactants.