**Core Concept**
Blindness is a significant visual impairment that affects an individual's ability to perform daily activities and interact with their environment. The World Health Organization (WHO) and the National Programme for Control of Blindness (NPCB) have established standardized definitions for blindness and low vision to facilitate global comparisons and interventions.

**Why the Correct Answer is Right**
The correct answer, **< 3/60 and < 3/60**, reflects the definitions of blindness established by the WHO and NPCB. According to these organizations, blindness is defined as a visual acuity of less than 3/60 in the better eye with the best possible correction. This threshold is used to categorize individuals with severe visual impairment, which significantly impacts their quality of life and independence. The WHO's definition of blindness is essential for global health initiatives and resource allocation, as it enables policymakers to identify areas with high prevalence of blindness and develop targeted interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option incorrectly pairs the NPCB's definition (< 3/60) with the WHO's definition (< 6/18), which are both used to define blindness but for different organizations.

**Option B:** This option reverses the correct definitions, incorrectly pairing the WHO's definition (< 6/18) with the NPCB's definition (< 3/60).

**Option D:** This option incorrectly pairs the same definition (< 6/18) for both the WHO and NPCB, which are not the correct definitions used by these organizations.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the definition of blindness can vary slightly between organizations and countries, but the WHO's definition of blindness as a visual acuity of less than 3/60 in the better eye is widely accepted and used globally.

**✓ Correct Answer: C. < 3/60 and < 3/60**