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 via the hepatic portal vein for further processing and distribution to the rest of the body.

Why Other Options Are Incorrect:

• A. Stomach:
• The stomach's primary role is to break down food mechanically and chemically (through gastric juices) but not to absorb nutrients. It mainly absorbs some water, alcohol, and certain medications, but the majority of nutrient absorption occurs in the small intestine.
• B. Pancreas:
• The pancreas produces digestive enzymes and bicarbonate to aid in digestion but does not absorb nutrients. Its role is crucial for digestion, but it does not have a direct function in nutrient absorption.
• D. Liver:
• The liver plays a vital role in processing and storing nutrients after they have been absorbed from the small intestine. However, it does not absorb nutrients itself; rather, it receives them from the bloodstream and processes them.

Summary of Key Points:

• The small intestine is the primary site for nutrient absorption in the digestive system.
• Its structure, with villi and microvilli, maximizes the surface area for absorption.
• Nutrients such as carbohydrates, proteins, and fats are absorbed into the bloodstream in their simplest forms.
• Other organs like the stomach, pancreas, and liver have important roles in digestion and processing but do not absorb nutrients directly.

This understanding of the small intestine's role in nutrient absorption is crucial for anyone studying biology, especially in the context of human physiology and nutrition.