The correct answer is D. Small intestine.
The small intestine is the longest part of the digestive system, and it is where most of the absorption of nutrients occurs. The small intestine is about 20 feet long, and it is divided into three parts: the duodenum, jejunum, and ileum.
The duodenum is the first part of the small intestine, and it is where the stomach contents are mixed with bile and pancreatic juices. Bile is a fluid that helps to break down fats, and pancreatic juices contain enzymes that help to break down carbohydrates, proteins, and fats.
The jejunum is the middle part of the small intestine, and it is where most of the absorption of nutrients occurs. The jejunum has a large surface area, which is important for absorption. The jejunum also has many villi, which are small finger-like projections that increase the surface area even more.
The ileum is the last part of the small intestine, and it is where bile salts and vitamin B12 are absorbed. The ileum also has a large surface area, but it does not have as many villi as the jejunum.
After the nutrients are absorbed, they are transported to the bloodstream and then to the cells of the body. The nutrients are used by the cells for energy, growth, and repair.