The process that takes place in the ileum is:

The ileum, which is the final section of the small intestine, plays a crucial role in the digestive system, particularly in the absorption of nutrients. After food is digested in the preceding sections of the small intestine, namely the duodenum and jejunum, the ileum is primarily responsible for finalizing the absorption process.

In the ileum, the lining is equipped with villi and microvilli, which greatly increase the surface area and enhance the efficiency of nutrient absorption. Essential nutrients such as vitamins, minerals, and amino acids are absorbed into the bloodstream through the intestinal walls in this section. Additionally, bile salts and certain sugars are also absorbed here.

While digestion does occur earlier in the digestive process and excretion refers to the elimination of waste products, the specific function of the ileum centers on absorption rather than these processes. Fermentation typically occurs in the large intestine, where certain substances are broken down by bacterial action, diverging from the primary role of the ileum. Hence, the correct answer emphasizes the ileum's defining function in nutrient absorption.