Correct Option: C. Small intestine Detailed Explanation: The small intestine is the primary organ responsible for the absorption of nutrients in the digestive system. Here’s a step-by-step breakdown of why this is the case:

1. Structure of the Small Intestine:
2. The small intestine is a long, coiled tube that is approximately 20 feet (6 meters) in length in adults. It is divided into three parts: the duodenum, jejunum, and ileum.
3. The inner surface of the small intestine is lined with tiny, finger-like projections called villi, which increase the surface area for absorption. Each villus is covered with even smaller hair-like structures called microvilli, forming a brush border that further enhances absorption.
4. Function of the Small Intestine:
5. The primary function of the small intestine is to digest food and absorb nutrients. After food is broken down in the stomach, it enters the small intestine where it is mixed with digestive enzymes and bile.
6. The small intestine is where most of the digestion and absorption of nutrients occurs. Carbohydrates, proteins, and fats are broken down into their simplest forms (sugars, amino acids, and fatty acids) and absorbed through the walls of the intestine into the bloodstream.
7. Nutrient Absorption:
8. Nutrients absorbed in the small intestine include:
   • Carbohydrates: Broken down into simple sugars (like glucose) and absorbed into the bloodstream.
   • Proteins: Broken down into amino acids, which are then absorbed.
   • Fats: Emulsified by bile and broken down into fatty acids and glycerol, which are absorbed through the intestinal walls.
9. The absorbed nutrients are then transported to the liver and other parts of the body for use.

Why Other Options Are Incorrect:

• A. Stomach:
• The stomach's primary role is to break down food mechanically and chemically using gastric juices, which contain hydrochloric acid and digestive enzymes. While some absorption occurs in the stomach (like alcohol and certain medications), it is not the main site for nutrient absorption. The stomach mainly prepares food for further digestion in the small intestine.
• B. Pancreas:
• The pancreas is an accessory organ that produces digestive enzymes and bicarbonate to neutralize stomach acid. It plays a crucial role in digestion but does not absorb nutrients itself. Instead, it releases enzymes into the small intestine to aid in the digestion of carbohydrates, proteins, and fats.
• D. Large intestine:
• The large intestine (or colon) primarily absorbs water and electrolytes from indigestible food matter and compacts waste into feces. While it does absorb some vitamins produced by gut bacteria (like vitamin K), it is not responsible for the absorption of the majority of nutrients, which occurs in the small intestine.

Summary of Key Points:

• The small intestine is the main site for nutrient absorption in the digestive system.
• It has a large surface area due to villi and microvilli, enhancing its ability to absorb nutrients.
• The stomach and pancreas play roles in digestion but not in nutrient absorption.
• The large intestine mainly absorbs water and electrolytes, not nutrients.

This understanding of the digestive system is crucial for grasping how our bodies utilize the food we consume, making the small intestine a vital organ in the process.