**Core Concept**
The World Health Organization (WHO) defines **blindness** based on visual acuity, which is a measure of the sharpness of vision. This definition is crucial for epidemiological studies and healthcare planning. The WHO definition is widely accepted and used internationally.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's definition of blindness, which states that blindness is present when the visual acuity is less than 3/60 or corresponding visual field loss in the better eye with best possible correction. This definition takes into account the severity of visual impairment and provides a standardized threshold for classification.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately reflect the WHO's definition of blindness.
**Option B:** Incorrect as it is not the correct visual acuity threshold.
**Option D:** Incorrect because it is not related to the WHO definition.

**Clinical Pearl / High-Yield Fact**
The WHO definition of blindness is essential for healthcare professionals to understand, as it helps in identifying individuals who require assistance and support. It is also crucial for planning and implementing blindness prevention programs.

**Correct Answer:** D. Visual acuity of less than 3/60.