**Core Concept**
The intestinal motility is regulated by a complex interplay of the enteric nervous system, hormones, and the autonomic nervous system. Certain substances can modulate intestinal motility by influencing these regulatory mechanisms.

**Why the Correct Answer is Right**
The correct answer is **Option D: Cholestyramine**. Cholestyramine is a bile acid sequestrant that decreases intestinal motility by binding to bile acids in the intestine, thereby reducing the enterohepatic circulation of bile acids. This action decreases the stimulation of the enteric nervous system, leading to a decrease in intestinal motility. Additionally, cholestyramine can also bind to other substances that stimulate intestinal motility, such as serotonin, and reduce their activity.

**Why Each Wrong Option is Incorrect**
**Option A:** Atropine is an anticholinergic agent that actually increases intestinal motility by blocking the action of acetylcholine, a neurotransmitter that decreases intestinal motility.

**Option B:** Metoclopramide increases gastric motility, but its effect on intestinal motility is less clear and can vary depending on the situation.

**Option C:** This option is not relevant to the question.

**Clinical Pearl / High-Yield Fact**
Cholestyramine's effect on intestinal motility is an important consideration in patients with certain gastrointestinal conditions, such as constipation or diarrhea.

**Correct Answer:** D. Cholestyramine