**Core Concept**
The myenteric plexus, also known as Auerbach's plexus, is a component of the enteric nervous system (ENS) that plays a crucial role in regulating gastrointestinal (GI) tract motility. It is a network of nerve fibers located between the inner circular and outer longitudinal layers of smooth muscle in the GI tract.

**Why the Correct Answer is Right**
The myenteric plexus controls GI tract motility by regulating the contraction and relaxation of smooth muscle cells. This is achieved through the release of neurotransmitters such as acetylcholine and nitric oxide, which act on muscarinic and nicotinic receptors, respectively. The myenteric plexus also regulates the coordination of peristalsis, ensuring that food is propelled through the GI tract in a coordinated and efficient manner.

**Why Each Wrong Option is Incorrect**
**Option A:** The submucosal plexus (Meissner's plexus) is another component of the ENS, but it primarily regulates secretory and blood flow functions, not motility.
**Option B:** The sympathetic nervous system can influence GI tract motility, but it is not directly controlled by the myenteric plexus.
**Option C:** The enteric nervous system as a whole is responsible for the "little brain" function of the GI tract, but the myenteric plexus specifically controls motility.

**Clinical Pearl / High-Yield Fact**
The myenteric plexus is often affected in conditions such as Hirschsprung's disease, which is characterized by the absence of ganglion cells in the distal colon and rectum, leading to severe constipation and megacolon.

**Correct Answer:** D.