The correct answer is option 3, hydrochloric acid. Hydrochloric acid is synthesized in the human stomach as part of the digestive process. It is produced by the parietal cells located in the lining of the stomach. Hydrochloric acid plays an important role in digestion by helping to break down food and kill harmful microorganisms that may be present in the stomach. It also activates enzymes that are necessary for the digestion of proteins. Sulphuric acid (option 1), nitric acid (option 2), and phosphoric acid (option 4) are not synthesized in the human stomach. Sulphuric acid is primarily used in industrial processes, nitric acid is used in the production of fertilizers and explosives, and phosphoric acid is used in food products and cleaning agents.