Answer:
Lower esophageal sphincter
Explanation:
The biological valves are the muscular structures composed of the smooth muscles which form thick tubular structure to allow the controlled movement of any material through it. The valves in the digestive system are known as the sphincters.
The sphincters control the unidirectional movement of the eaten food, chyme and the bolus in the digestive tract.
A sphincter is present at the lower portion of the oesophagus (food pipe) which contracts to prevent the backflow of the food from the stomach and since the sphincter is present at the lower side of the oesophagus, therefore, is known as the lower esophagal sphincter.
Thus, Lower esophageal sphincter is correct.
