Let us analyze the change in the oxidation state of iron in each process:

1. Rusting of iron sheets: Iron reacts with oxygen and moisture to form rust, which is hydrated iron(III) oxide ($\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$). Here, the oxidation state of $\text{Fe}$ changes from $0$ to $+3$. This is an oxidation process.
2. Decolourisation of blue $\text{CuSO}_4$ solution by iron: Iron displaces copper from copper sulphate solution: $\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$. Here, the oxidation state of $\text{Fe}$ changes from $0$ to $+2$. This is an oxidation process.
3. Formation of $\text{Fe}(\text{CO})_5$ from $\text{Fe}$: In the metal carbonyl complex pentacarbonyliron(0), $\text{Fe}(\text{CO})_5$, the ligand carbon monoxide ($\text{CO}$) is a neutral molecule. Therefore, the oxidation state of iron remains $0$. This process does not involve the oxidation of iron.
4. Liberation of $\text{H}_2$ from steam by iron at high temperature: Iron reacts with steam to form iron(II,III) oxide ($\text{Fe}_3\text{O}_4$) and hydrogen gas: $3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$. Here, the oxidation state of $\text{Fe}$ changes from $0$ to an average of $+8/3$ (a mixture of $+2$ and $+3$). This is an oxidation process.