This question tests your ability to incorporate information from a passage about a fictional organism with many structural and functional similarities to humans to answering a question about similar cellular functions between the two organisms. Ultimately, you are told in the question stem that the new organism contains cells within the stomach called "betts cells" that are very functionally similar to chief cells in the human stomach. Thus, the question is testing your knowledge of the function of human chief cells. The question specifically asks to choose the substance that is produced directly by betts cells, which essentially is asking which substance is produced directly by human chief cells. The only substance listed that is directly produced by the chief cells in the stomach is pepsinogen, which is a zymogen, meaning that it is a precursor to pepsin. Pepsinogen is activated to pepsin when it meets acid in the stomach. Because pepsinogen requires activation outside of the chief cells in order to become pepsin, pepsin is not directly produced by the chief cells, making pepsin an incorrect answer to the question.

Hydrochloric acid is produced by the parietal cells of the stomach, not chief cells.

Gastrin is produced by G cells in the stomach and duodenum, but not chief cells.

Thrombin is a clotting factor found in blood. It is not produce by chief cells.

Calcium plays an important role in many physiological functions, including signal transduction, muscle contraction, and bone formation. Of all of the processes listed in the answer choices, calcium is least directly related to biotin and vitamin K production. Biotin and vitamin K are synthesized by gut flora.  

This question asks us to understand osmolarity and osmosis. Osmosis is the property of water whereby it moves from a high concentration of water to a low concentration of water, while osmolarity refers to the tendency of solutes to affect the draw of water from one area to another. In more practical terms, this means that water moves from areas of low solute concentration to areas of high solute concentration. Fiber is not digestible by humans, thus it cannot be absorbed by the gastrointestinal enterocytes. Additionally, because fiber cannot be absorbed, it cannot affect the absorption of other compounds in the gut to a great extent. Fiber does, however, increase the number of solute particles in the gut, and so draws water from the circulation and tissues via osmosis. Increasing the flow of water into the lumen will add more water to stool and help relieve constipation.

Fat-soluble vitamins include A, D, E, and K. Water-soluble vitamins include B and C. 

While carbohydrates and proteins are digested and absorbed directly into the bloodstream, dietary fats are packaged into chylomicrons and moved into the lacteal. The lymphatic system then transports the dietary fats to the large veins of the body via the thoracic duct. The majority of fats are absorbed by this process, but a smaller minority of shorter fatty acids are able to float freely into the bloodstream.

The majority of food absorption occurs in the later two-thirds of the small intestine, the jejunum and the ileum. Water and water-soluble vitamins, however, are absorbed in the large intestine.

The colon extracts water and salts while forming feces. The colon also absorbs some nutrients that made it through the small intestine without being absorbed, but its main job is water and salt absorbtion. 

The primary purpose of the colon is to reabsorb water and salt (NaCl) in order to dehydrate the stool into a compact form for defecation. On average, the colon reabsorbs 1.5L of water per day and moves excreted sodium from the stool back into systemic circulation for further use. Inhibition of sodium reabsorption by the cholera toxin is what leads to severe dehydration and diarrhea seen in the disease.

The primary roles of the large intestine are water and electrolyte absorption. If this role is impeded by a pathogen, diarrhea results. Dehydration can also be a secondary effect of large intestine damage, and can lead to death.

The small intestine is primarily involved in nutrient absorption and chemical digestion. The stomach is involved in mechanical digestion, and some chemical digestion. The liver produces bile, which can be secreted to emulsify fats.

The posterior abdominal wall is the location of many organs, one of which is the transverse colon. This part of the colon brings waste from one side of the body to the other, and hangs along the posterior wall in the abdominal cavity.

The colon consists of a few main parts. The ascending colon is the first region of the large intestine, connecting it to the small intestine. The ascending colon transitions into the transverse colon, which then becomes the descending colon. The descending colon transitions into the muscular sigmoidal colon, which terminates in the rectum.