**Core Concept**
Optic nerve injury following sinus surgery is a serious complication that can occur due to the proximity of the optic nerve to certain ethmoidal cells. The optic nerve passes close to the sphenoid sinus and the Onodi cells, which are a type of posterior ethmoidal cell. These cells are located in a critical area where the optic nerve is at risk of injury during surgical procedures.

**Why the Correct Answer is Right**
Onodi cells are a type of posterior ethmoidal cell that is located in close proximity to the optic nerve. During sinus surgery, the removal of these cells can lead to injury of the optic nerve, resulting in vision loss or other complications. The Onodi cells are specifically defined as posterior ethmoidal cells that have a posterior wall that is in contact with or is less than 2 mm from the optic nerve. This close proximity to the optic nerve makes the Onodi cells a high-risk area for optic nerve injury during surgery.

**Why Each Wrong Option is Incorrect**
**Option A:** Haller cells are a type of maxillary sinus cell that is located in the anterior wall of the maxillary sinus. While they can be a risk factor for maxillary sinusitis, they are not typically associated with optic nerve injury following sinus surgery.

**Option B:** Agger nasi cells are a type of anterior ethmoidal cell that is located in the anterior wall of the ethmoid sinus. They are not typically associated with optic nerve injury following sinus surgery.

**Option D:** Bullae cells are a type of maxillary sinus cell that is located in the anterior wall of the maxillary sinus. They are not typically associated with optic nerve injury following sinus surgery.

**Clinical Pearl / High-Yield Fact**
To avoid optic nerve injury during sinus surgery, it is essential to identify and preserve the Onodi cells. This can be done by using high-resolution imaging studies and careful surgical technique. The "Onodi cell sign" is a radiological feature that can indicate the presence of an Onodi cell, and surgeons should be aware of this sign to avoid complications.

**✓ Correct Answer: C. Onodi cells**