When a single element undergoes simultaneous oxidation and reduction in a reaction to give two or more products showing different change in oxidation state is called a Disproportionation reaction. For a molecule to undergo disproportionation reaction the number of free or valence electrons should have suitable electronic configuration to give more the one oxidation states.

In option (i), Carbon only undergoes combustion to give carbon dioxide and water, this does not give more than one carbon products and for that of oxygen the oxidation number is same.

For option (ii) there is only free radical substitution reaction of hydrogen getting replaced by chlorine so there is no change in oxidation state of carbon or chlorine.

For option (iii) the compound under consideration contains fluorine, but according to rule for oxidation numbers, fluorine cannot attain any other oxidation number than -1, so disproportionation reaction are not possible with fluorine compounds.

Thus option (iv) is correct, the disproportionation reaction can be shown as follows: