What moves the food from your mouth to your stomach when you swallow? undulatory muscle contractions pressure difference between zones peristaltic muscle contractions gravity?

The appropriate answer is the peristaltic muscle muscle contractions.
Peristalsis, involves involuntary movements of the longitudinal and circular muscles, primarily in the digestive tract but may take place in other hollow tubes of the body, that occur in progressive wavelike contractions. Peristaltic waves occur in the esophagus, stomach, and intestines.

The correct answer is peristaltic muscle contractions.
  Peristaltic muscle contractions include contraction and relaxation of smooth muscles that produce a peristaltic wave, which propels the bolus (mix of food and saliva which travels down the digestive tract). Muscles contract behind the bolus to prevent it from being squeezed back into the mouth and the rhythmic contractions work to rapidly force it into the stomach.