Final answer:
The top/first portion of the small intestine is called the duodenum, which is essential for digestion with enzymatic secretion. Following is the jejunum for nutrient absorption and finally the ileum, which also absorbs nutrients and connects to the large intestine. So, the correct answer is option d.
Step-by-step explanation:
The top/first portion of the small intestine is called the duodenum. This is an important area where the digestion of food intensifies significantly with the aid of enzymes secreted from the digestive juices. The duodenum is the shortest and the initial segment of the small intestine, paving the way for further digestion and absorption which occurs down the line in other parts of the intestine.
The jejunum is the middle section of the small intestine, following the duodenum. It's the site of extensive nutrient absorption, with carbohydrates and amino acids primarily absorbed here.
The last part of the small intestine is called the ileum, which also takes part in the absorption of nutrients including vitamin B12 and bile salts. The ileum concludes the small intestine and connects to the large intestine at the ileocecal valve.
So, the correct answer is option d.