## **Core Concept**
The World Health Organization (WHO) has guidelines to prevent the spread of diseases like malaria and yellow fever, which are transmitted by mosquitoes. A key strategy involves ensuring that areas around ports (airports and seaports) are free of mosquitoes to prevent the spread of these diseases. This is crucial for international health regulations.

## **Why the Correct Answer is Right**
The correct distance recommended by international health organizations, including the WHO, around airports and seaports that should be free of mosquitoes is **400 meters**. This distance is chosen because it is generally considered sufficient to prevent the spread of mosquito-borne diseases from infected areas to ports, thereby reducing the risk of international transmission. Mosquitoes typically do not fly far, and this distance helps ensure that the risk of disease transmission is significantly reduced.

## **Why Each Wrong Option is Incorrect**
- **Option A: 100 meters** - This distance is too short to effectively prevent the spread of mosquito-borne diseases, given that some mosquitoes can travel farther than this.
- **Option B: 200 meters** - Similarly, this is also on the shorter side for ensuring complete protection against mosquito-borne disease transmission.
- **Option D: 1 km** - While a larger area might offer more protection, **400 meters** is specifically recommended by health organizations as a practical and effective distance.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **400 meters** radius around airports and seaports free of mosquitoes is a critical preventive measure against the spread of vector-borne diseases. This is part of broader international health regulations aimed at controlling and preventing the spread of diseases across borders.

## **Correct Answer:** C. 400 meters.