The crypts of Lieberkuhn are small tubular structures found in the lining of the small intestine. They are named after the German anatomist, Johann Nathanael Lieberkuhn, who first described them in the 18th century. These crypts play an essential role in the digestion and absorption of nutrients in the small intestine.

The crypts of Lieberkuhn secrete a fluid called the succus entericus, which contains various enzymes and hormones that aid in digestion. Among the given options, the correct answer is option 'D', succus entericus.

Now, let's understand the function of each option to see why succus entericus is the correct answer:

a) Gastrin: Gastrin is a hormone produced by the stomach lining. It stimulates the release of gastric acid, which aids in the digestion of food in the stomach. However, it is not secreted by the crypts of Lieberkuhn.

b) Rennin: Rennin, also known as chymosin, is an enzyme found in the stomach of infants. It helps in the coagulation of milk proteins, aiding in their digestion. It is not secreted by the crypts of Lieberkuhn.

c) Cholecystokinin: Cholecystokinin (CCK) is a hormone produced by the cells of the small intestine and released in response to the presence of fat and protein. It stimulates the release of digestive enzymes from the pancreas and the contraction of the gallbladder. Although CCK is involved in digestion, it is not secreted by the crypts of Lieberkuhn.

d) Succus entericus: Succus entericus, also known as intestinal juice, is a fluid secreted by the crypts of Lieberkuhn. It contains various digestive enzymes, such as amylase, lipase, and peptidases, which help in the breakdown of carbohydrates, fats, and proteins, respectively. Additionally, succus entericus contains hormones like secretin, which stimulates the release of bicarbonate-rich pancreatic juice, and enterogastrone, which inhibits gastric motility and secretion. Succus entericus is crucial for the final stages of digestion and nutrient absorption in the small intestine.

In conclusion, the correct answer is option 'D', succus entericus, as it is the fluid secreted by the crypts of Lieberkuhn and contains digestive enzymes and hormones necessary for the digestion and absorption of nutrients in the small intestine.

While the chyme is present within the small intestine, glands within this structure begin to secrete different types of digestive enzymes called carbohydrases (amylase, proteases, and lipases or fat splitting enzymes). The glands responsible for these enzymatic secretions are called the crypts of Lieberkuhn. Another set of glands called Brunner's glands are responsible for the secretion of mucus. Collectively, the intestinal secretions are referred to as succus entericus.