Final answer:
The correct answer is the muscular layer, as it consists of inner circular and outer longitudinal smooth muscles that contract during peristalsis, facilitating the movement of food through the GI tract. The submucosa and serosa provide support but do not contribute to motility.
Step-by-step explanation:
The muscularis consists of two types of smooth muscle: an inner circular layer and an outer longitudinal layer. These muscle layers contract to propel food through the tract by the process of peristalsis. The muscular external layer is essential for mechanical digestion and the movement of food along the GI tract. In contrast, the submucosa connects the mucosa to the underlying muscularis, providing support but not motility, and the serosa is the outermost layer, which does not contribute to motility either. Hence, the correct answer to the question is a. muscular.