What is least involved in the mechanical breakdown of food

The esophagus is least involved in the mechanical breakdown of food.

Explanation:

Esophagus plays a least complex role in the breakdown of food, both anatomically and physiologically. Anatomically structured between the mouth and the stomach, this is a thin muscular channel which channels or propels the semi-solid food bolus from the mouth to the stomach through its unique rhythmic peristaltic movements and to the pull of gravity. There is no mechanical or chemical breakdown of food in the esophagus and none of the nutrients are absorbed. During digestion, after the initial breakdown in mouth, the next step takes place only in the stomach.

b. lithosphere, asthenosphere, mesosphere   is the answer most likely