**Core Concept**
Vitamin-A deficiency (VAD) is a significant public health concern worldwide, particularly in developing countries. The assessment of VAD in a community involves evaluating the biochemical, clinical, or nutritional indicators of deficiency.

**Why the Correct Answer is Right**
The correct answer for assessing VAD prevalence is Night Blindness (NYD), which is a clinical indicator of vitamin-A deficiency. Night blindness is caused by a decrease in rhodopsin, the pigment in the retina responsible for vision in low light conditions. Vitamin A is essential for the regeneration of rhodopsin, and its deficiency leads to impaired visual adaptation in dim light. The World Health Organization (WHO) recommends the assessment of night blindness as a practical and cost-effective method for identifying vitamin-A deficiency in communities.

**Why Each Wrong Option is Incorrect**
* **Option A:** Serum retinol levels are a biochemical indicator of vitamin-A status, but they are not a direct measure of deficiency prevalence. Serum retinol levels can fluctuate due to various factors, such as recent intake of vitamin A-rich foods or supplements.
* **Option B:** Hepatic vitamin A stores can be an indicator of vitamin-A status, but they are not a direct measure of deficiency prevalence. Hepatic stores can be depleted before clinical manifestations of deficiency appear.
* **Option C:** Serum transthyretin levels are not directly related to vitamin-A status. Transthyretin is a transport protein for thyroxine and vitamin A, but its levels do not reflect vitamin-A deficiency.

**Clinical Pearl / High-Yield Fact**
The WHO recommends the assessment of night blindness, along with other clinical indicators, such as xerophthalmia and conjunctival xerosis, to diagnose vitamin-A deficiency in communities. These indicators are essential for identifying populations at risk and planning interventions to prevent and control vitamin-A deficiency.

**Correct Answer: B. Serum retinol levels are a biochemical indicator of vitamin-A status, but they are not a direct measure of deficiency prevalence. Serum retinol levels can fluctuate due to various factors, such as recent intake of vitamin A-rich foods or supplements.