**Core Concept**
The pupillary reflex, also known as the pupillary light reflex, is a complex process that involves the coordination of multiple cranial nerves and brainstem nuclei to regulate pupil size in response to changes in light intensity. This reflex is crucial for adapting to varying light conditions and maintaining proper vision.

**Why the Correct Answer is Right**
The pupillary reflex is primarily controlled by the Edinger-Westphal nucleus (a parasympathetic nucleus) in the midbrain, which receives input from the optic nerve (CN II) and sends fibers to the ciliary ganglion. The Edinger-Westphal nucleus releases acetylcholine, which stimulates the sphincter pupillae muscle to constrict the pupil. This process is mediated by the muscarinic receptors in the sphincter pupillae muscle. In addition, the pretectal nuclei in the midbrain also play a role in integrating the pupillary reflex, particularly in response to light stimulation.

**Why Each Wrong Option is Incorrect**
* **Option A:** The oculomotor nerve (CN III) is involved in pupillary reflex but is not the primary controlling nucleus. It carries the parasympathetic fibers from the Edinger-Westphal nucleus to the sphincter pupillae muscle.
* **Option B:** The trochlear nerve (CN IV) is responsible for innervating the superior oblique muscle, which is involved in eye movement, not pupillary reflex.
* **Option D:** The trigeminal nerve (CN V) is primarily responsible for sensory input from the face and is not directly involved in the pupillary reflex.

**Clinical Pearl / High-Yield Fact**
The pupillary reflex is an important clinical indicator of midbrain function and can be assessed in patients with suspected neurologic disorders, such as third cranial nerve palsy or midbrain lesions.

**Correct Answer: C. Edinger-Westphal nucleus**