**Core Concept**
The length of the eyeball at birth is an essential anatomical measurement that varies significantly from the adult length. This is due to the continuous growth and development of the eye throughout childhood.

**Why the Correct Answer is Right**
The length of the eyeball at birth is approximately 16-17 mm. This measurement is crucial for ophthalmologists to assess the normal development of the eye and to diagnose any abnormalities or conditions such as microphthalmia. The eyeball length increases gradually as the child grows, reaching its adult length of around 24 mm by the age of 10 years.

**Why Each Wrong Option is Incorrect**
**Option A:** 12 mm - This is too short for a normal eyeball at birth.
**Option B:** 20 mm - This is closer to the adult length, but still not accurate for a newborn.
**Option C:** 18 mm - While closer, this is still not the correct average length at birth.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the length of the eyeball can be affected by various conditions, such as microphthalmia, nanophthalmia, or coloboma. Accurate measurement and assessment of the eyeball length are critical for diagnosing and managing these conditions.

**Correct Answer: B. 16-17 mm**