Which of the following structures make up the small intestine?

The small intestine is composed of three primary sections: the duodenum, jejunum, and ileum. Among the given options, the duodenum is a crucial part of the small intestine, as it is the first section that receives chyme (partially digested food) from the stomach, alongside bile from the liver and digestive enzymes from the pancreas. This combination of fluids is essential for the continued digestion and absorption of nutrients.

The colon, pancreas, and stomach, while important components of the digestive system, are not considered parts of the small intestine. The colon is part of the large intestine, which primarily absorbs water and salts, whereas the pancreas is an organ that produces enzymes vital for digestion, and the stomach is responsible for breaking down food before it enters the small intestine. Thus, the correct answer highlights the significant role of the duodenum in the small intestine's function.