When pure water boils vigorously, the bubbles that rise to the surface

When pure water boils vigorously, the bubbles that rise to the surface are composed primarily of

air
hydrogen
hydrogen and oxygen
water vapour
This question was previously asked in
UPSC NDA-2 – 2017
When pure water boils vigorously, the process involves the phase transition of liquid water into gaseous water.
Boiling occurs when the vapour pressure of the liquid equals the surrounding atmospheric pressure, and the liquid turns into gas within the bulk of the liquid as well as at the surface. The bubbles formed during boiling are filled with the gaseous form of the liquid, which in this case is water vapour.
Air might be present as dissolved gas in water, and tiny air bubbles might be released upon heating before boiling, but the large, vigorous bubbles during boiling are predominantly water vapour. Water molecules (H₂O) decompose into hydrogen and oxygen only under extreme conditions (like electrolysis), not during simple boiling.