Which of the following structures is part of the small intestine?

The jejunum is indeed part of the small intestine, as it is one of its three primary sections, along with the duodenum and ileum. The small intestine plays a crucial role in digestion and nutrient absorption. The jejunum specifically is where a significant portion of nutrient absorption occurs, particularly for carbohydrates and proteins.

The other structures mentioned are not components of the small intestine. The duodenum is actually the first segment of the small intestine, following the stomach. The cecum is a pouch connecting the small intestine to the large intestine, and the colon is a part of the large intestine itself. Therefore, while the duodenum is correctly associated with the small intestine, the jejunum is also a correct part of this digestive system, emphasizing the various sections involved in digestion and absorption.