The absorption of usable nutrients occurs in the

Small intestine

Explanation:

Around eighty percent of usable nutrients are absorbed in the small intestine.

The main area of gastrointestinal tract responsible for absorption is small intestine. It consists of three sections, i.e. the duodenum, jejunum and ileum. In duodenum, jejunum  majority of nutrients are absorbed.

The duodenum absorbs carbohydrates, proteins, and starches that are in the form of glucose or proteins are absorbed in the jejunum.