**Core Concept**
The muscles responsible for facial expressions, particularly smiling, are primarily innervated by the facial nerve (cranial nerve VII) and are crucial for conveying emotions and social interactions. The primary muscles involved in smiling are responsible for elevating the corners of the mouth and forming a curvature of the lips.

**Why the Correct Answer is Right**
The zygomaticus major muscle is a broad, fan-shaped muscle that originates from the zygomatic bone and inserts into the skin of the upper lip and the orbicularis oris muscle. During smiling, the zygomaticus major muscle contracts, pulling the corners of the mouth upwards and laterally, creating a characteristic smile. This muscle is responsible for the primary action of smiling, and its contraction is often associated with feelings of happiness and joy.

**Why Each Wrong Option is Incorrect**
**Option A:** The levator labii superioris alaeque nasi muscle is involved in elevating the upper lip and the ala of the nose, but it is not primarily responsible for smiling.
**Option B:** The orbicularis oris muscle surrounds the mouth and is responsible for closing the lips, but it is not the primary muscle involved in smiling.
**Option C:** The buccinator muscle is involved in compressing the cheeks against the teeth and is not primarily responsible for smiling.

**Clinical Pearl / High-Yield Fact**
The facial nerve (cranial nerve VII) innervates the muscles of facial expression, including the zygomaticus major muscle, and is responsible for conveying emotions and social interactions through facial expressions.

**Correct Answer:** C. Zygomaticus major muscle.