**Core Concept**
Chyme, a semi-fluid mixture of food and digestive enzymes, undergoes mechanical and chemical digestion in the small intestine. The propulsion of chyme is essential for the efficient breakdown and absorption of nutrients.

**Why the Correct Answer is Right**
The movement of chyme in the small intestine is primarily facilitated by peristalsis, a wave-like muscular contraction that moves intestinal contents forward. This process is mediated by the enteric nervous system and involves the contraction of smooth muscle cells in the intestinal wall. The peristaltic waves are triggered by the presence of chyme and help to mix it with pancreatic juice, bile, and intestinal enzymes, ensuring optimal digestion and absorption of nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because the movement of chyme in the small intestine is not primarily facilitated by the muscularis mucosae, a thin layer of smooth muscle in the mucosa that is involved in the movement of the mucosa itself, not the propulsion of chyme.

**Option B:**
This option is incorrect because the movement of chyme in the small intestine is not primarily facilitated by the cilia of the epithelial cells lining the small intestine, which are involved in the movement of mucus and debris, not the propulsion of chyme.

**Option C:**
This option is incorrect because the movement of chyme in the small intestine is not primarily facilitated by the contraction of the diaphragm, which is involved in breathing and the movement of the abdominal organs, not the propulsion of chyme.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the small intestine is a long, narrow tube with a large surface area, allowing for efficient absorption of nutrients. The peristaltic waves in the small intestine are essential for mixing chyme with digestive enzymes and ensuring optimal digestion and absorption of nutrients.

**Correct Answer: D. Peristalsis.**