Doubt on stoichiometry

Q)11
Answer is option 3.

CO2 is absorved by KOH. So if 20ml contraction is observed, 20ml was CO2 out of the 90ml.
When you explode something, you get combustion, where your end products are water (here in vapour state), and carbon dioxide. So the rest 70ml is water.

From this, and the vapour density you can quite easily get the composition.