The esophagus is part of the:

The esophagus is indeed a part of the alimentary canal, which is a continuous tube that extends from the mouth to the anus, facilitating the process of digestion and the movement of food. The alimentary canal includes several organs such as the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus. The esophagus specifically functions as a conduit for food and liquids to travel from the throat to the stomach. Its muscular walls help push the food downward through a series of contractions called peristalsis.

The other options pertain to different anatomical or physiological systems. The abdominal wall refers to the layer of muscles, tendons, and skin covering the abdominal cavity. A mucous membrane lines various cavities and surfaces of organs within the body, primarily for protection and secretion, but it is not a structural system on its own. The circulatory system, which encompasses the heart and blood vessels, is responsible for the circulation of blood and nutrients throughout the body, unrelated to the digestive function of the esophagus.