Baking soda is chemically sodium hydrogen carbonate (NaHCO₃). However, the options provided describe a mixture, and option C specifically lists sodium hydrogen carbonate and tartaric acid, which is the primary composition of baking powder, a common leavening agent used in baking. While pure baking soda is just sodium hydrogen carbonate (a compound, not a mixture), the question likely intends to refer to a common baking mix or is slightly misphrased, with option C being the closest correct description of a relevant mixture.