• Digestion of protein is initiated in the stomach by the enzyme pepsin.
  
• In the small intestine, further digestion is done by trypsin, chymotrypsin, and carboxypeptidase into dipeptidase that releases from the pancreas.
• Dipeptidase are broken into amino acids by intestinal dipeptidase.
  
• These amino acids are absorbed actively by walls of intestine and release in the bloodstream.
• These are utilized by tissues and broken down into carbon dioxide and ammonia.
• Proteins require maximum energy to digest.

 Parietal cells of stomach secrets hydrochloric acid which convert inactive pepsinogen to partially active pepsinogen, partially activates pepsinogen then cleaves the peptide from other pepsinogen molecules to form active pepsin which converts protein into proteases and peptides.

(i) Pepsin digest protein in the stomach.

A) Amylase is produced by buccal cavity, pancreas and intestine and digest starch.

If pH of the stomach is made 7 protein digestion will affect as pepsin works ar pH of 2 to 3 and enzymes are highly precise about there function so it does not activate.

The chief function of HCl in the stomatch is to maintain low pH to activate pepsinogen to form pepsin. HCl converts inactive pepsinogen into the active pepsin enzyme. The pepsin helps in the breaking down proteins in the stomach. HCl also kills bacteria that enter the stomach. It also helps in digestion of  other foods in the stomach.

The HCl is secreted by the oxyntic cells in the stomach. The HCl provides the acidic medium in the stomach that is responsible for converting the inactive pepsinogen to the active pepsin. Pepsin is required for protein digestion. The acidic medium provides defence against bacteria entering the digestive system.

• Protein digestion occurs in the stomach and duodenum in which 3 main enzymes: Pepsin, trypsin and chymotrypsin.
•  pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. 
• Hence, Lipase enzymes are not useful in the digestion of proteins.
• So, the correct answer is 'Lipase'.

The digestion of the food starts at the mouth where the enzyme amylase is secreted which digests the start that is present in the food and converts it into maltose. From there through the esophagus the food arrives in the stomach where it is stored for 4 hours and mixed with acid. The food when arrives at the stomach where the wall of the stomach secrets HCl with a pH of 1.8. This acid activates the inactive pepsinogen enzyme and forms active pepsin which digests proteins and converts them into proteases and peptides. From here the food moves to the small intestine where secretion from the liver and pancreas reach through a hepatopancreatic duct which is alkaline in nature and neutralizes the acidic food that comes from the stomach. Here majority of the digestion of protein to amino acids, fat to fat droplets, polysaccharides to monosaccharides etc takes place.

In the stomach, proteins are hydrolysed by enzyme pepsin. It begins the digestion of proteins by splitting them into peptones (see peptidase). It is produced by the action of $HCl\left( { H }^{ + } \right) $ on pepsinogen, which is secreted by the stomach in gastric glands.

Only some chemicals medicines and alcohol are absorbed in oro pharyngeal cavity. In the stomach, absorption of water, some salts, alcohol and glucose takes place, complete absorption of alcohol takes place in the stomach. Thus, carbohydrates, proteins and fats will remain in alimentary canal for complete digestion and when broken down completely in small intestine then only they will be absorbed in the intestine but beer will not be digested in alimentary canal and directly absorbed in the stomach.

Carbohydrate digestion starts in the mouth as salivary amylase is present there. Protein digestion starts in the stomach as protein digesting enzymes is present in stomach and fat digestion occurs in the small intestine.

Rennin is a milk-curdling enzyme, that occurs in the gastric juices of infants and is also contained in the rennet produced in the stomach of calves and other ruminants. It is an endopeptidase, that converts casein to paracasein and was formerly used extensively as a curdling agent by the cheese industry. 

Stomach in vertebrates is the main site for digestion of proteins. Pepsin is the chief proteolytic enzyme of stomach secreted in the inactive form called as pepsinogen. The pepsinogen is activated to pepsin in presence of hydrochloric acid secreted in stomach by oxyntic or parietal cells. The optimum pH for activity of pepsin is 1.8.

Hydrochloric acid (HCl) converts pepsinogen to pepsin which breaks down proteins to peptides. HCl maintains a pH in the stomach of approximately 2.0. It also dissolves food and kills microorganisms. Without enough HCl, pepsinogen is no converted to pepsin which do not allow the breakdown of proteins

Stomach in vertebrates is the main site for digestion of proteins. Pepsin is the chief proteolytic enzyme of stomach secreted in the inactive form called as pepsinogen. The pepsinogen is activated to pepsin in presence of hydrochloric acid secreted in stomach by oxyntic or parietal cells. The optimum pH for activity of pepsin is 1.8.

In the presence of enterokinase, inactive trypsinogen is converted into active trypsin.

Gastric juice comprising a mixture of substances, including pepsin and hydrochloric acid, secreted by glands of the stomach. Its principal function is to break down proteins into polypeptides during digestion.

Pepsin is the chief digestive enzyme in the stomach that stomach that breaks down proteins. Cells produce pepsinogen that is produced to act on mucus by epithelial cells of the stomach that helps to protect the lining of the stomach from corrosive hydrochloric acid and pepsin.

• Digestion involves the mixing of food, its movement through the digestive tract, and the chemical breakdown of the large molecules of food into smaller molecules.
  
•  Digestion begins in the mouth when we chew and swallow, and then it is passed on to the stomach where digestion takes place by gastric juices and enzymes secreted by the stomach and then it is passed to the small intestine where the digestion is completed. 
• Pepsin is an enzyme which is released by zymogenic cells, the chief cells in the stomach and that degrades food proteins into peptides. Hence, the stomach is the structure where the protein digestion first occurs.

• Pepsin is an enzyme which is released by zymogenic cells, the chief cells in the stomach and that degrades food proteins into peptides.
•  It is most active in acidic environments.
•  It exhibits maximal activity at pH 2.0 and is inactive at pH 6.5. Hence, Pepsin is an enzyme which acts in an acidic medium in the stomach.
  

Pepsin acts in 'acidic medium'.

If the pH of the stomach is 1.6, then 'pepsin' will digest protein.

Digestion of 'proteins' will be affected if the pH of the stomach is made 7.

Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological function in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Histamine is involved in the inflammatory response and has a central role as a mediator of itching. 

Chyme is the name given to the partially digested food, formed in the stomach and enters into the small intestine(duodenum) via the pyloric valve. Chyme, also known as chymus has the consistency of oatmeal(semisolid). Chyme is a thick semisolid mass of partially digested food and digestive secretions during the digestion process. Hence, partially digested semisolid food in the stomach is chyme.

Pepsin is produced in the stomach and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Zymogenic cell or peptic cell is a cell in the stomach that releases pepsinogen and chymosin.

The proenzyme pepsinogen(inactive), on exposure to hydrochloric acid, gets converted to the active enzyme pepsin. Pepsin hydrolyses proteins to polypeptide chains and initiates protein digestion in the digestion process in humans. Hence, the pepsin digests protein in the stomach.

So, the correct option is 'proteins in the stomach'.