Answer: b) is waves of muscular contractions that move along the length of the digestive tract.
Step-by-step explanation:
a) is not right, since the movement is triggered by a signal sent to the brain, which coordinates the movements.
c) is not right, since the esophagus smooth muscle is arranged in circular and longitudinal muscle layers.
NOTE: the peristalsis do not only ocurrs in the esophagus. It also occurs in all the digestive tract, as well as in the bile conducts and also in the urinary system, to conduct the urine from the kidneys to the bladder.