Explanation:

- Baking soda is the common name for sodium bicarbonate, which has the chemical formula NaHCO3.

- Option 1: Hydrogen

- Hydrogen is present in baking soda as part of the bicarbonate ion (HCO3–).

- Option 2: Calcium

- Calcium is not a component of baking soda.

- It is sometimes confused with calcium carbonate, which is a different compound.

- Option 3: Sodium

- Sodium is a key component of baking soda, hence the name sodium bicarbonate.

- Option 4: Oxygen

- Oxygen is present in baking soda as part of both the carbonate and bicarbonate ions.

Option 2: Calcium is the correct answer.