**Core Concept**
Vitamin-A deficiency is a significant public health concern, especially in developing countries, leading to night blindness, xerophthalmia, and increased mortality in children. The assessment of Vitamin-A deficiency prevalence in a community is crucial for designing targeted interventions.

**Why the Correct Answer is Right**
The correct method to assess Vitamin-A deficiency prevalence is through Night Blindness (NB) survey, also known as the "Dark Adaptation Test". This involves assessing the ability of children to adapt to darkness after being exposed to a bright light. The test is based on the principle that Vitamin-A deficiency affects the functioning of the retina, particularly the rod cells, leading to impaired dark adaptation. The World Health Organization (WHO) recommends the Night Blindness survey as a rapid and cost-effective method to assess Vitamin-A deficiency prevalence in a community.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum retinol levels are not a reliable indicator of Vitamin-A deficiency prevalence, as serum levels may remain normal even in the presence of deficiency.
**Option B:** The 24-hour urine retinol test is not a practical or reliable method for assessing Vitamin-A deficiency prevalence in a community.
**Option C:** The Schirmer test measures tear production and is not directly related to Vitamin-A deficiency.

**Clinical Pearl / High-Yield Fact**
The Night Blindness survey is a simple, cost-effective, and reliable method for assessing Vitamin-A deficiency prevalence in a community, making it an essential tool for public health programs.

**Correct Answer: C. Schirmer test is used to assess tear production and is not related to Vitamin-A deficiency.**