**Core Concept**
Applanation tonometry is a method used to measure intraocular pressure (IOP) by flattening a small area of the cornea with a given force. The principle relies on the assumption that the pressure inside the eye is proportional to the force required to flatten a given area of the cornea. This technique is crucial in diagnosing and managing conditions like glaucoma.

**Why the Correct Answer is Right**
The correct answer is likely related to the accuracy of applanation tonometry. One of the most significant sources of error in applanation tonometry is the corneal thickness. If the cornea is too thick or too thin, it can affect the accuracy of the measurement. However, another important source of error is the rigidity of the cornea, which can be affected by various conditions such as keratoconus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a widely recognized source of error in applanation tonometry. However, without more information, it's hard to pinpoint the exact reason.
* **Option B:** This option is incorrect because it's not directly related to the accuracy of applanation tonometry. While corneal diseases can affect the measurement, they are not typically considered a source of error in the technique itself.
* **Option C:** This option is incorrect because it's not a recognized source of error in applanation tonometry. However, it's possible that it's related to a different aspect of ophthalmology.

**Clinical Pearl / High-Yield Fact**
A key aspect of applanation tonometry is to ensure that the cornea is not too thick or too thin, as this can affect the accuracy of the measurement. A corneal thickness of around 550-600 microns is considered optimal for applanation tonometry.

**Correct Answer: C.**