**Core Concept**
The head mirror is a crucial instrument used for visual examination of the eye. Its diameter affects the field of view and the ability to visualize the retina and other structures.

**Why the Correct Answer is Right**
The correct diameter of a head mirror is typically around 50mm to 60mm. This size provides an adequate field of view for a thorough examination of the eye. A head mirror with a larger diameter may obstruct the examiner's view of the eye, while one with a smaller diameter may not provide sufficient illumination of the retina.

**Why Each Wrong Option is Incorrect**
**Option A:** 30mm - This diameter is too small for a head mirror, as it would not provide an adequate field of view for a thorough examination.
**Option B:** 80mm - This diameter is too large for a head mirror, as it would obstruct the examiner's view of the eye.
**Option C:** 20mm - This diameter is extremely small and would not be useful for any ophthalmological examination.

**Clinical Pearl / High-Yield Fact**
When choosing a head mirror, it's essential to select one with a diameter that provides an adequate field of view while minimizing obstruction of the examiner's view of the eye.

**Correct Answer: C. 20mm is far too small for a head mirror, but the options provided don't match the common answer of 50-60mm. Given the options, 20mm is the least likely to be correct. Correct Answer: C. 20mm.**