A sphincter is

A) a thickening of the longitudinal layer of the muscularis that propels materials through the GI tract.

B) fold of the mucosa of the GI tract that increases the total surface area for absorption of nutrients.

C) circle of adventitia that anchors the GI tract to neighboring structures.

D) thickening of the circular layer of the muscularis that controls movement of materials through the GI tract

Answer:option D.

Thickening of the circular layer of muscularis that controls movement of materials through the GI tract.

Explanation:

Sphincter are circular muscles that control the opening and closing of body parts. Their main function is to control the movement of materials or fluid in the body.

There are five sphincter of the digestive system and they are;

Pyloric sphincter.

The lower oesophageal sphincter.

Ilocecal sphincter

Internal sphincter

External sphincter.

All this sphincter function to control the passage or movement of materials through the GI tracts.