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 also absorbed into the bloodstream.
   • Fats: Emulsified by bile and broken down into fatty acids and glycerol, which are absorbed into the lymphatic system before entering the bloodstream.
9. Role of Enzymes and Bile:
10. The pancreas secretes digestive enzymes into the small intestine that help break down carbohydrates, proteins, and fats.
11. The liver produces bile, which is stored in the gallbladder and released into the small intestine to help emulsify fats, making them easier to digest and absorb.

Why Other Options Are Incorrect:

• A. Stomach:
• The stomach is primarily responsible for the mechanical and chemical digestion of food. It secretes gastric juices that break down food into a semi-liquid form called chyme. However, the stomach is not the site of nutrient absorption; it mainly prepares food for further digestion in the small intestine.
• B. Pancreas:
• The pancreas produces digestive enzymes and bicarbonate to neutralize stomach acid, but it does not absorb nutrients. Its role is crucial in digestion, but it does not directly participate in the absorption process.
• D. Large intestine:
• The large intestine (or colon) is primarily involved in the absorption of water and electrolytes, as well as the formation and excretion of feces. While it does absorb some vitamins produced by gut bacteria (like vitamin K), it is not the main site for nutrient absorption.

Summary of Key Points:

• The small intestine is the primary organ for nutrient absorption in the digestive system.
• It has a large surface area due to villi and microvilli, which facilitate the absorption of carbohydrates, proteins, and fats.
• The pancreas and liver play supportive roles by providing enzymes and bile, but they do not absorb nutrients themselves.
• The stomach and large intestine have different functions related to digestion and water absorption, respectively, but are not the main sites for nutrient absorption.

This understanding of the digestive system is crucial for grasping how our bodies utilize the food we eat, making the small intestine a key player in nutrition and health.