**Core Concept**
Snellen's chart is a widely used tool in ophthalmology for assessing visual acuity, which is a crucial aspect of eye examination. Visual acuity measures the sharpness of vision, enabling clinicians to evaluate the overall health of the eye and identify potential abnormalities.

**Why the Correct Answer is Right**
Snellen's chart consists of a series of high-contrast letters or symbols of varying sizes, which are arranged in a specific pattern. When a patient is asked to read the letters on the chart from a standard distance, their ability to identify the smallest letters correctly reflects their visual acuity. This test is essential for diagnosing and monitoring conditions such as myopia, hyperopia, and astigmatism. The chart is calibrated to ensure that a person with normal vision can read the largest letters (6/6 or 20/20) from a distance of 6 meters (20 feet), while those with impaired vision can only read smaller letters.

**Why Each Wrong Option is Incorrect**
**Option B:** Snellen's chart is not used to test refraction, which involves assessing the eye's ability to focus light and is typically evaluated using a phoropter or retinoscope.
**Option C:** While presbyopia, a age-related loss of near vision, can be assessed using Snellen's chart, the chart itself is not designed specifically for this purpose.
**Option D:** Colour blindness is tested using the Ishihara test or other coloured plates, not Snellen's chart.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Snellen's chart is not a substitute for a comprehensive eye examination, which should include a thorough medical history, visual acuity assessment, and other tests to evaluate the overall health of the eye.

**✓ Correct Answer: A. Vision**