Step 1: Understanding baking powder composition.
Baking powder typically contains sodium bicarbonate (sodium hydrogen carbonate), potassium hydrogen tartrate (cream of tartar), and starch. Sodium carbonate is not a typical component of baking powder.
Step 2: Analyzing the options.
(A) Sodium carbonate: This is not present in baking powder.
(B) Sodium hydrogen carbonate: This is the main ingredient in baking powder that reacts with acid to release CO\(_2\).
(C) Potassium hydrogen tartrate: This is cream of tartar, which is commonly used in baking powder.
(D) Starch: Starch is used as a stabilizer in baking powder to prevent premature reactions.
Step 3: Conclusion.
The correct answer is (A) Sodium carbonate.