$A$ catalyst in a chemical reaction

Which one of the following reactions is an example of heterogeneous catalysis?

$A$ catalyst is a substance that . . . .

Catalysts in the following reactions are:
$(I) CH_3COOCH_{3(l)} + H_2O_{(l)} \rightarrow CH_3COOH_{(aq)} + CH_3OH_{(l)}$
$(II) 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$ (Contact Process)
$(III) 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$ (Lead Chamber Process)
$(IV) N_{2(g)} + 3H_{2(g)} \rightarrow 2NH_{3(g)}$ (Haber Process)

In which one of the following processes do the reactants and catalyst exist in three different states?

Platinised asbestos helps in the formation of $SO_3$ from $SO_2$ and $O_2$. But,if even a small amount of $As_2O_3$ is present,the platinised asbestos does not help in the formation of $SO_3$. $As_2O_3$ acts here as