**Core Concept**
The submucosal plexus, also known as Meissner's plexus, is a complex network of neurons and glial cells located in the submucosa layer of the gastrointestinal tract (GIT). It plays a crucial role in regulating gut motility, secretion, and blood flow.

**Why the Correct Answer is Right**
The submucosal plexus is responsible for the parasympathetic innervation of the GIT, primarily through the release of acetylcholine, which acts on muscarinic receptors to stimulate smooth muscle contraction and glandular secretion. This plexus is also involved in the release of various neurotransmitters, such as substance P and vasoactive intestinal peptide (VIP), which regulate gut blood flow and secretion. The submucosal plexus works in conjunction with the myenteric plexus (Auerbach's plexus) to maintain normal gut function.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the submucosal plexus is not primarily responsible for sympathetic innervation of the GIT.
**Option B:** Incorrect because the myenteric plexus (Auerbach's plexus) is located between the inner circular and outer longitudinal layers of smooth muscle, not in the submucosa.
**Option C:** Incorrect because the enteric nervous system is a complex network that includes both the submucosal and myenteric plexuses, but this option is too vague to be correct.

**Clinical Pearl / High-Yield Fact**
The submucosal plexus is often affected in conditions such as irritable bowel syndrome (IBS), where abnormal gut motility and secretion can lead to symptoms of abdominal pain and altered bowel habits.

**Correct Answer:** D.