Final answer:
Acid production in the stomach is primarily stimulated by the hormone gastrin; other factors listed either contribute to the process of producing acid or are involved in regulation and digestion further down in the small intestine.
Step-by-step explanation:
Acid production in the stomach is stimulated by several factors, including hormones, neuronal signals, and the presence of food. Among the options provided, gastrin is a hormone that unequivocally stimulates the secretion of hydrochloric acid (HCl) by the parietal cells of the stomach mucosa. Additionally, while the number of H+/K+ pumps on the basolateral membrane of the parietal cells does contribute to the capacity of acid secretion, it is not a stimulatory factor in itself. The regulation of acid production involves a feedback mechanism where the stomach's own acid production can be inhibited when the pH gets too low, this being a protective measure rather than a stimulatory one. Moreover, the anion exchanger on the apical membrane of the parietal cells is involved in the exchange of chloride ions with bicarbonate, aiding in the acidification process but not directly stimulating acid production. Activation of trypsinogen, converting it to trypsin, occurs in the small intestine and is related to protein digestion rather than stimulating acid production in the stomach.