**Core Concept**
The underlying principle being tested is the regulation of gastrointestinal motility, specifically the process of peristalsis. Peristalsis is controlled by the enteric nervous system and involves the coordinated contraction and relaxation of intestinal smooth muscle.

**Why the Correct Answer is Right**
The correct substance is likely to be involved in the enteric nervous system's control over peristalsis, potentially affecting the direction or force of muscular contractions. However, without the specific options provided, we can infer that substances influencing gut motility, such as neurotransmitters or hormones, play a crucial role.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, it's challenging to provide a detailed explanation, but typically, incorrect options might include substances not directly involved in gut motility or those that do not cause retrograde contraction.
**Option B:** Similarly, this option would be incorrect if it does not directly influence peristalsis in a retrograde manner.
**Option C:** This option might be incorrect if it is known to cause antegrade (forward) peristalsis rather than retrograde.
**Option D:** Assuming this is not the correct answer, it would be a substance that does not significantly contribute to retrograde contraction during peristalsis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that retrograde peristalsis, or the movement of intestinal contents in the opposite direction to normal peristalsis, can be an important mechanism in certain physiological or pathological states, such as vomiting or the retrograde movement of contents into the esophagus in gastroesophageal reflux disease.

**Correct Answer:** Correct Answer: D. Substance P