**Core Concept**
The head mirror is an essential diagnostic tool in ENT (Ear, Nose, and Throat) outpatient clinics, allowing clinicians to visualize the ear canal, tympanic membrane, and other structures in the head and neck region. The focal length of the head mirror is critical in determining its ability to magnify these structures.

**Why the Correct Answer is Right**
The correct answer is 14-16 cm. This focal length allows for optimal magnification of the ear canal and tympanic membrane, enabling ENT specialists to diagnose conditions such as otitis media, perforated eardrums, and middle ear infections. The head mirror's focal length is designed to be close enough to the ear to provide a clear view of the tympanic membrane without causing discomfort or distortion.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because a focal length of 10-12 cm would result in insufficient magnification of the ear canal and tympanic membrane.
**Option B:** Incorrect because a focal length of 18-20 cm would cause the head mirror to be too far away from the ear, leading to a distorted view.
**Option C:** Incorrect because a focal length of 6-8 cm would be too short, causing the head mirror to be uncomfortable to wear and potentially causing eye strain.

**Clinical Pearl / High-Yield Fact**
When using a head mirror in ENT-OPD, it's essential to adjust the focal length to ensure optimal magnification of the ear canal and tympanic membrane. A focal length of 14-16 cm is generally considered ideal for this purpose.

**Correct Answer: D. 14-16 cm**