**Core Concept**
Vision 2020 is a global initiative to eliminate avoidable blindness by the year 2020. Visual acuity is a critical parameter to assess the visual function of an individual, and under Vision 2020, a simple and effective method is recommended for its assessment in school children.

**Why the Correct Answer is Right**
The correct answer refers to the use of a Snellen's chart, which is a widely accepted and standardized tool for measuring visual acuity. The Snellen's chart has a series of high-contrast letters on a white background, with the letters placed at a decreasing size as the chart progresses. The chart is placed at a distance of 6 meters from the child, and the child is asked to read the letters in the chart. The visual acuity is then calculated based on the size of the smallest letter that the child can read correctly.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be incorrect, as it does not specify a valid method for assessing visual acuity in school children under Vision 2020.
**Option B:** This option might be incorrect, as it does not specify a valid method for assessing visual acuity in school children under Vision 2020.
**Option C:** This option might be incorrect, as it does not specify a valid method for assessing visual acuity in school children under Vision 2020.

**Clinical Pearl / High-Yield Fact**
The Snellen's chart is a widely accepted and standardized tool for measuring visual acuity, and it is essential to use it in a well-lit room with the chart placed at a distance of 6 meters from the child.

**Correct Answer: D. A Snellen's chart is used to check visual acuity in school children under Vision 2020.