What is ratio of hydrogen and oxygen in water by volume? 

2:1

The correct answer is option 3. 2:1.

The chemical name for water is H₂O. Water is electrolyzed to produce gases of hydrogen and oxygen, as seen below:

2H₂O(l) → 2H₂(g) + O₂(g)

By electrolyzing water, hydrogen and oxygen are generated in quantities that are 2:1. Because each water molecule has one oxygen atom and two hydrogen atoms, this is the case.