To determine which process does not involve the oxidation of iron, we need to analyze each option.$\\$Option 1: Decolourization of blue $\mathrm{CuSO}_4$ solution by iron.$\\$• In this process, iron (Fe) displaces copper (Cu) from $\mathrm{CuSO}_4$ solution.$\\$• The reaction is: $\\$ $\mathrm{Fe} + \mathrm{CuSO}_4 \rightarrow \mathrm{FeSO}_4 + \mathrm{Cu} \\$• Iron is oxidized from $\mathrm{Fe^0}$ to $\mathrm{Fe^{2+}}. \\$Option 2: Formation of $\mathrm{Fe(CO)}_5$ from Fe.$\\$• In this process, iron reacts with carbon monoxide (CO) to form iron pentacarbonyl.$\\$• The reaction is: $\\$ $\mathrm{Fe} + 5\mathrm{CO} \rightarrow \mathrm{Fe(CO)}_5 \\$• Iron remains in the zero oxidation state (Fe$^0$) in $\mathrm{Fe(CO)}_5. \\$• Therefore, there is no oxidation of iron.$\\$Option 3: Liberation of $\mathrm{H}_2$ from steam by iron at high temperature.$\\$• In this process, iron reacts with steam to produce hydrogen gas and iron oxide.$\\$• The reaction is: $\\$ $3\mathrm{Fe} + 4\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Fe}_3\mathrm{O}_4 + 4\mathrm{H}_2 \\$• Iron is oxidized from $\mathrm{Fe^0}$ to $\mathrm{Fe^{3+}}. \\$Option 4: Rusting of iron sheets.$\\$• Rusting involves the oxidation of iron in the presence of oxygen and moisture.$\\$• The reaction is: $\\$ $4\mathrm{Fe} + 3\mathrm{O}_2 + 6\mathrm{H}_2\mathrm{O} \rightarrow 4\mathrm{Fe(OH)}_3 \\$• Iron is oxidized from $\mathrm{Fe^0}$ to $\mathrm{Fe^{3+}}. \\$Conclusion:$\\$• Option 2, the formation of $\mathrm{Fe(CO)}_5$ from Fe, does not involve oxidation of iron.$\\$• Therefore, the correct option is 2.