**Core Concept**
The nasolacrimal duct is a critical structure in the lacrimal apparatus, responsible for draining tears from the eye into the nasal cavity. Its anatomy is essential for understanding the pathophysiology of conditions such as epiphora (excessive tearing) and dacryocystitis (inflammation of the lacrimal sac).

**Why the Correct Answer is Right**
The nasolacrimal duct measures approximately 12 mm in length, extending from the lacrimal sac to the inferior meatus of the nasal cavity. This duct is lined by mucous membrane and has a cartilaginous and bony component, which allows it to maintain its patency and facilitate tear drainage. The length of the nasolacrimal duct is crucial for the proper functioning of the lacrimal apparatus and is often affected in conditions such as nasolacrimal duct obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 mm - This is too short to accurately represent the length of the nasolacrimal duct.
**Option B:** 11 mm - While close, this measurement is still shorter than the actual length of the duct.
**Option D:** 9 mm - This is significantly shorter than the correct length and would not allow for proper tear drainage.

**Clinical Pearl / High-Yield Fact**
The nasolacrimal duct is often affected in conditions such as nasolacrimal duct obstruction, which can lead to epiphora and dacryocystitis. It is essential to understand the anatomy of the lacrimal apparatus to diagnose and manage these conditions effectively.

**✓ Correct Answer: C. 12 mm**