**Core Concept**
The National Leprosy Elimination Programme (NLEP) aims to control and eventually eliminate leprosy from India. Mass surveys are a crucial component of this program, helping to identify new cases and monitor disease prevalence.

**Why the Correct Answer is Right**
Mass surveys are conducted in areas with a high prevalence of leprosy, typically defined as more than 1 case per 10,000 population. This threshold triggers targeted interventions, including enhanced surveillance, case finding, and health education. The prevalence rate serves as an indicator of the disease burden, guiding public health strategies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the actual prevalence threshold for mass surveys under NLEP is not specified here.
**Option B:** This option is incorrect because it is not the correct prevalence threshold for mass surveys under NLEP.

**Clinical Pearl / High-Yield Fact**
It's essential to recall that the NLEP's goal is to achieve a prevalence rate of less than 1 case per 10,000 population, indicating a significant reduction in leprosy burden.

**Correct Answer:** 1 case per 10,000 population.