At nearly 70°C, sodium bicarbonate undergoes a decomposition reaction, which means it breaks down into different products. In the case of sodium bicarbonate used in bakery products, the product responsible for its use is carbon dioxide.

Option 1: Carbon dioxide is the correct answer. When sodium bicarbonate decomposes, it produces carbon dioxide gas as one of the products. This gas is what causes the dough to rise in baked goods, creating a lighter and fluffier texture.

Option 2: Hydrogen is not the correct answer. While sodium bicarbonate does release some hydrogen gas during decomposition, it is not the primary product and is not directly responsible for the usefulness in bakery products.

Option 3: Water vapor is not the correct answer. Water vapor is not a product of the decomposition of sodium bicarbonate. The reaction mainly produces carbon dioxide gas, not water vapor.

Option 4: Oxygen is not the correct answer. Oxygen is not a product of the decomposition of sodium bicarbonate. The main product is carbon dioxide gas, which is responsible for the desired effect in bakery products.

In conclusion, the correct answer is option 1, carbon dioxide.