The puffed up appearance of dough is due to the production of:

The correct answer is Option (2) → CO₂ gas

Option (1) Oxygen gas — Incorrect. Fermentation goes on without oxygen and does not release it.

Option (2) CO₂ gas — Correct. Bacteria (in dosa and idli batter) or baker's yeast (in bread) ferment the sugars of the dough and give off CO₂.

Option (3) Ethyl alcohol — Incorrect. Some alcohol forms too, but a liquid cannot make the dough rise.

Option (4) Pyruvic acid — Incorrect. Pyruvic acid is a step in the breakdown of glucose inside the cell; it does not leave the cell as a gas.

NCERT Extract:
Microbes in Household Products – “… The dough, which is used for making foods such as dosa and idli is also fermented by bacteria. The puffed-up appearance of dough is due to the production of CO₂ gas.”