As far as I can tell, there is no such thing as a bolus, tracheal, or salivary sphincter. There are two sphincters that esophageal sphincter refers to, which is the upper and the lower.
The upper esophageal sphincter prevents food that has been eaten from going into the air passageway and into the lungs. While this doesn't describe what your question wants, the lower esophageal sphincter does.
The lower esophageal sphincter is located at the very end of the esophagus right before the point at which food goes into the stomach. It prevents stomach acid and food in the stomach from coming back up into the esophagus. This will be your answer.
