## Core Concept
The question pertains to the allocation of a female health worker in plains based on population size. This relates to the organizational structure and resource allocation in public health, specifically within the context of primary health care services.

## Why the Correct Answer is Right
The correct answer, , indicates that for a population of 30,000 in plains, a female health worker is allocated. This allocation is based on the norms set for public health services, aiming to ensure accessible healthcare to the population. The population size of 30,000 is a standard benchmark used in many health systems for planning and delivering primary healthcare services, including the deployment of auxiliary nurse midwives (ANMs) or female health workers.

## Why Each Wrong Option is Incorrect
- **Option A:** is incorrect because it specifies a much larger population (1,20,000) that would likely require more than one health worker for effective coverage.
- **Option B:** is incorrect as it mentions 18,000, which, although closer, does not match the standard allocation norm of 30,000 for plains areas.
- **Option D:** is incorrect because 50,000 exceeds the population size typically allocated to one female health worker in plains, potentially leading to inadequate healthcare coverage.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the allocation of healthcare workers, including female health workers, is crucial for ensuring equitable access to healthcare services. In India, for instance, the norm is to have one Auxiliary Nurse Midwife (ANM) for every 30,000 population in plains and 20,000 in hilly, tribal, or backward areas. This information can be critical for planning, policy-making, and evaluating healthcare delivery systems.

## Correct Answer: C. 30,000.