The correct answer is Stomach.

Mistake Points

• The enzyme pepsin digests proteins and, Chief cells of stomach secrete enzyme pepsin along with HCL So the digestion of protein begins in stomach.  

Key Points

• Protein is one of most important substances in body.
• The digestion of protein begins in  stomach.
• Chief cells or peptic cells of stomach secrete proenzyme pepsinogen. The proenzyme pepsinogen on exposure to hydrochloric acid gets converted into active enzyme pepsin
• The enzyme pepsin is responsible for digestion of proteins.
• Pepsin converts proteins into proteases and peptones.

Important Points

• Parietal or Oxyntic cells of  stomach secrete HCL ad intrinsic factor 

Additional Information

• The liver produces bile to help break down and absorb fats.
• Waste products and toxins are removed through bile.
• The small intestine is part of intestines where 90% of digestion and absorption of food occurs.
• The body recycles amino acids to make more proteins.

• The liver is largest gland in body.
• The liver mainly secretes 'Bile Juice' which is stored in Gall bladder.
• Bile Juice and Pancreatic juice are released into small intestine by a common duct.
• Unused glucose is stored in liver in form of glycogen.
• Heparin, Urea, and Bile Juice are produced in liver.
• The major supply of blood to liver is by 'portal vein'(75%) & remaining (25%) by Hepatic artery.
• That is why liver is known to have a 'Dual blood supply'.
   

The correct answer is Small intestine.

Key Points

• The small intestine is longest part of digestive system.
• The length of small intestine is 7 meters (22 feet).
• The main functions of small intestine are to complete digestion of food and to absorb nutrients. 
• Passage of food through small intestine normally takes three to six hours.
• Bile, pancreatic enzymes, and alkaline juice (HCO3–) are three main substances that are received by small intestine.
• Approximately 80% of water is absorbed by small intestine, 10% of water is absorbed by large intestine and remaining 10% excreted in faeces.
• The small intestine is divided into duodenum, jejunum, and ileum.
• Iron is absorbed in duodenum, most are absorbed in jejunum and Vitamin B12 and bile salts are absorbed in latter part of ileum.