The puffed-up appearance of dosa and idli is due to the production -

The correct answer is Option (2) → Carbon dioxide

The dough for dosa and idli is fermented by bacteria. The bacteria break down sugars in the dough and release a gas, and the gas puffs up the dough.

(1) Carbon monoxide — Incorrect. This poisonous gas is not a product of fermentation of dough.

(2) Carbon dioxide — Correct. The bacteria produce CO₂ gas. It gets trapped in the dough and gives it the puffed-up look. Baker’s yeast does the same in bread dough.

(3) Methane — Incorrect. Methane is made by methanogenic bacteria in biogas plants and in the gut of cattle. It is not made in dough.

(4) Hydrogen — Incorrect. Hydrogen is not the gas that makes dough rise.

Note: A similar question was also asked in CUET (UG) 2025 on 23-05-2025 (Slot 2), and in CUET (UG) 2023 on 23-06-2023 (Slot 3).

NCERT Extract:
Microbes in Household Products – …