(B) 2 : 1
During electrolysis of water, hydrogen is liberated at the cathode and oxygen at the anode in the volume ratio of 2 : 1, as water molecule contains two hydrogen atoms for every one oxygen atom.
The chemical equation for electrolysis of water is: 2H₂O → 2H₂ + O₂. Since 2 volumes of H₂ are produced for every 1 volume of O₂, the ratio is 2:1. Note: the source passages provided do not directly cover this topic, but the answer is based on standard Class 10 Chemistry (Chapter 1 — Chemical Reactions and Equations / practical knowledge). Examiners expect students to recall this fact directly.