Answer: D. Hydrochloric acid
Step-by-step explanation:
The hydrochloric acid is produced by the cells present on the lining of the stomach walls. The function of the hydrochloric acid in the stomach is to provide an acidic environment for the digestion of food. The acidity in the stomach is responsible for the activation of the enzymes which supports the digestion of proteins present in the food stuff. Also the acidic environment facilitates the death of the disease causing pathogens like bacteria. This way the hydrochloric acid plays role in providing the internal immunity to the body.