This is the glycol analogue of the glycerol-oxalic acid reaction (which gives allyl alcohol at about 260 degrees C): the vicinal diol is deoxygenated to the alkene.
Ethylene glycol (\(\mathrm{HOCH_2CH_2OH}\)) heated with oxalic acid at about 210 \(^\circ\)C forms a cyclic oxalate ester intermediate, which loses \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\) (analogous to glycerol + oxalic acid giving allyl alcohol at 260 \(^\circ\)C): the vicinal diol is converted into the alkene. \(\mathrm{HOCH_2CH_2OH} \xrightarrow{\text{oxalic acid, } 210^\circ C} \mathrm{CH_2{=}CH_2} + \mathrm{CO_2} + \mathrm{CO} + \text{H}_2\mathrm{O}\) (net deoxygenation of the glycol). Vinyl alcohol (option 2) is unstable (it tautomerises to acetaldehyde), and glycolaldehyde and glyoxal (options 1 and 4) are oxidation products, which oxalic acid does not give here. So X = ethene. Answer: (3) CH2=CH2
