The volume ratio of hydrogen and oxygen gases liberated during electrolysis of water is:

In Activity 1.7 of the NCERT chapter, water is electrolysed using two carbon electrodes connected to a 6 V battery, with dilute sulphuric acid added to make it conducting. The chapter states that bubbles form at both electrodes and that 'the volume of the gas collected' is not the same in both test tubes — one collects double the volume of the other. On testing with a burning candle, the gas collected in smaller volume (at the anode) is oxygen, which supports combustion, while the gas collected in double the volume (at the cathode) is hydrogen, which burns with a pop. This is because water, $H_2O$, contains hydrogen and oxygen atoms in the ratio 2:1, so electrolysis liberates hydrogen and oxygen gases in the same 2:1 volume ratio.

Why the other options are wrong:

(A) 1:2 reverses the actual ratio — it would mean more oxygen than hydrogen, which contradicts the composition of water ($H_2O$ has twice as many H atoms as O atoms).

(C) 1:3 and (D) 4:1 do not correspond to the atomic composition of water or the observation described in Activity 1.7 that one test tube collects exactly double the gas of the other.