In manufacture of bread, it becomes porous due to the liberation of CO2 by the action of

The correct option is "Yeast".

Here is the step-by-step educational explanation of why yeast is responsible for making bread porous:

1. What is Yeast?
Yeast is a microscopic, single-celled eukaryotic fungus. The most common species used in baking bread is Saccharomyces cerevisiae, commonly known as baker's yeast.

2. The Process of Fermentation
When yeast is mixed with flour, water, and sugar to make bread dough, it starts feeding on the sugars present in the dough. In the absence or limited presence of oxygen within the dense dough, yeast undergoes anaerobic respiration, which is also known as alcoholic fermentation. The chemical equation for this fermentation process can be represented as:
$$C_6{H}_{12}{O}_6\to 2C_2{H}_{5}OH+2C{O}_2$$
(Glucose ⇒ Ethanol + Carbon Dioxide)

3. Why Bread Becomes Porous
During fermentation, carbon dioxide (CO₂) gas is liberated. The sticky, elastic gluten network in the dough traps this released CO₂ gas. As the gas bubbles expand, they cause the dough to rise (leaven). When the dough is baked, the high heat causes the trapped gas bubbles to expand further before the proteins and starches set, and the alcohol evaporates. Once the bread is fully baked, it retains these empty pockets where the gas bubbles once were, giving the bread its characteristic soft, spongy, and porous texture.