**Core Concept**
The Caldwell-Luc operation is a surgical procedure used to access the maxillary sinus, typically performed to treat chronic maxillary sinusitis or other conditions affecting the maxillary sinus. This operation involves creating an opening in the anterior wall of the maxillary sinus through the canine fossa.

**Why the Correct Answer is Right**
The most common complication of the Caldwell-Luc operation is the formation of a mucocele in the maxillary sinus. A mucocele is a mucus-filled cavity that can occur when the normal drainage of the sinus is obstructed, leading to accumulation of mucus. This can happen due to the surgical alteration of the sinus anatomy or the creation of a new pathway for mucus to accumulate. The formation of a mucocele can lead to chronic sinusitis, facial pain, and other symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Orbital floor fracture is a potential complication of the Caldwell-Luc operation, but it is not the most common complication.
**Option B:** The Caldwell-Luc operation is not commonly associated with this complication, which is more related to other types of sinus surgeries.
**Option C:** While infection is a risk with any surgical procedure, it is not the most common complication specifically associated with the Caldwell-Luc operation.

**Clinical Pearl / High-Yield Fact**
To avoid complications like mucocele formation, it's essential for surgeons to carefully plan and execute the Caldwell-Luc operation, taking into account the delicate anatomy of the maxillary sinus and the potential risks of altering its normal drainage pathways.

**Correct Answer: D. Mucocele formation in the maxillary sinus.**