**Core Concept**
The development of the ear involves the formation of the external, middle, and inner ear structures. Each part of the ear develops at different rates and reaches adult size at various times. Understanding these developmental timelines is crucial for diagnosing and managing congenital ear anomalies.

**Why the Correct Answer is Right**
The external ear, or pinna (auricle), and the middle ear are generally fully formed and nearly adult size at birth. However, the inner ear structures, particularly the **cochlea** and **semicircular canals**, are fully formed early in fetal development but the **external auditory canal** and **tympanic membrane** are also nearly adult size. The **Eustachian tube** is relatively small and continues to develop postnatally.

**Why Each Wrong Option is Incorrect**
**Option A:** The external ear is one of the first structures to reach adult size.
**Option B:** The middle ear is also developed and nearly adult size at birth.
**Option C:** Not provided, however, the inner ear is fully formed but not all components related to it are fully developed.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **Eustachian tube** is smaller and more horizontal in children, which can lead to **middle ear effusions** and infections due to impaired drainage.

**Correct Answer:** D. Eustachian tube