Biology

The bubbles in bread come from carbon dioxide. During fermentation, yeast consumes sugars in the dough and produces carbon dioxide and ethanol. The carbon dioxide gas forms bubbles, which get trapped in the dough, causing it to rise and creating the airy texture of the bread.

The other options are incorrect:

Lactic acid is produced during lactic acid fermentation (not by yeast in bread making) and contributes to sourness in foods like yogurt or sauerkraut. It does not create bubbles.