Final answer:
b) Oblique
The third layer of smooth muscle in the stomach's wall is the oblique layer, which aids in the churning and mixing of food during digestion.
Step-by-step explanation:
The stomach has three layers of smooth muscles in its walls: the circular layer, the longitudinal layer, and the oblique layer. The muscularis externa, also known as the muscularis, is responsible for the peristalsis that moves food through the gastrointestinal (GI) tract. The stomach has a unique third layer of smooth muscle, the oblique layer, which is not found in other parts of the GI tract like the small intestine or colon. This oblique layer enables the stomach to vigorously churn and mix food as part of the digestion process.