The process that makes bread soft and spongy when baking soda (or NaHCO3 -sodium bicarbonate) is added involves a chemical reaction that produces carbon dioxide gas (CO2 - option 3). The gas gets trapped in the dough creating small bubbles or pockets. When the dough is baked, these gas pockets expand and create the fluffy, spongy texture we associate with bread.

As for other options, option 1, Sodium salt of acid, isn`t particularly relevant to the softness and sponginess of bread. Option 2, NaHCO3, is merely the chemical formula for baking soda - the agent used to produce CO2. Option 4, H2O which represents water, does play a role in making the dough but not in making it spongy. Therefore, the correct answer is 3, CO2, because it is the creation and expansion of CO2 gas during baking that creates the characteristic spongy texture of bread.