The small intestine in the human body consists of three structural parts: the duodenum, jejunum, and ileum.

Option 1: Jejunum—Duodenum—Ileum is incorrect because the jejunum, which is listed first, is not the longest part of the small intestine.

Option 2: Ileum—Duodenum—Jejunum is also incorrect because the duodenum, listed second, is the shortest part—not greater in length than the jejunum.

Option 3: Jejunum—Ileum—Duodenum is incorrect as well because, similar to Option 1, the jejunum is not the longest part of the small intestine.

Option 4: Ileum—Jejunum—Duodenum, which is the correct answer, correctly lists the parts of the small intestine in order of decreasing length. The ileum is generally the longest part, followed by the jejunum, and finally the duodenum.