Which of the following structures is part of the digestive system?

The stomach is correctly identified as part of the digestive system. It plays a crucial role in the process of digestion by breaking down food through mechanical churning and the secretion of gastric juices, which contain enzymes and acids. This process transforms food into a semi-liquid substance called chyme, which is then passed into the small intestine for further digestion and nutrient absorption.

In contrast, the kidney is part of the excretory system, responsible for filtering blood and producing urine. The bronchus is part of the respiratory system, facilitating airflow into the lungs, and the thymus is part of the immune system, playing a critical role in the development of T-cells. Understanding the distinct functions and roles of each of these systems is essential in recognizing how the body's organs collaborate to maintain overall health.