The reaction of ethylene (ethene) is:
C2H4 (g) + 3O2 (g) → 2CO2 (g) + 2H2O (g)
The molar ratio of C2H4 to water is 1molC2H4 : 2 mol H2O
The volumes of ideal gases at constant temperature and pressure are proportional to the number of mols.
Then, you can state that the volumetric ratio of the two gases is the same that above molar ratio:
1 Liter C2H4 : 2 Liter H2O
Given that you have 1.26 Liter C2H4, the volume of water is:
1.26 Liter C2H4 * [2 Liter H2O / 1 Liter C2H4] = 2.52 Liter H2O
Answer: 2.52 Liter of water.