Which structure keeps the acid chyme from spurting up esophagus as stomach undergoes muscular contractions?

The correct answer is Lower Esophageal Sphincter.

The lower esophageal sphincter (LES) is a muscular valve separating the esophagus from the stomach. When it is relaxed, it allows food to enter the stomach. When it is contracted, it ensures that food and digestive juices remain in the stomach and do not return to the esophagus. This is important in preventing acid reflux and heartburn.