Final answer:
HCl is necessary in the stomach for activating pepsin and killing bacteria, while mucous serves to protect the stomach lining from the corrosive effect of stomach acid by creating a protective barrier.
Step-by-step explanation:
The correct answer to the question is: b) HCl activates pepsin and kills bacteria, while mucous protects the stomach lining from acid. Hydrochloric acid (HCl) in the stomach serves vital functions in digestion, particularly in the breakdown of proteins. It helps to denature protein molecules, making them more accessible to the enzyme action of pepsin, which is activated by the acidic environment that HCl provides. Beyond digestion, HCl also plays a defensive role by killing potential pathogens found in foods.
On the other hand, mucous has an essential protective function in the stomach lining. The stomach mucosa produces a thick coating of bicarbonate-rich mucus that forms a barrier, preventing the stomach's own digestive enzymes from breaking down its tissues, which would lead to ulcers. This mucosal barrier is crucial for maintaining the integrity of the stomach lining against the corrosive acidic environment necessary for digestion.