**Core Concept**
The yellow fever control measures aim to prevent the spread of the disease, which is transmitted by the Aedes mosquito. The primary goal is to eliminate the breeding sites of the Aedes mosquito, particularly around airports where international travelers arrive from endemic areas.

**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends a 400-meter radius around airports to be kept free of Aedes breeding sites. This measure is crucial in preventing the introduction and spread of yellow fever virus among travelers and local populations. The Aedes mosquito breeds in stagnant water, and the 400-meter radius helps to ensure that no breeding sites are present around airports, minimizing the risk of yellow fever transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the actual distance recommended by WHO is 400 meters, not 200 meters. The 200-meter radius would not be sufficient to prevent Aedes breeding around airports.
**Option B:** This is incorrect because the recommended distance is 400 meters, not 600 meters. The 600-meter radius would be excessive and may not be feasible in all airport settings.
**Option C:** This is incorrect because the recommended distance is a fixed 400 meters, not a variable distance that depends on the airport's size or layout.

**Clinical Pearl / High-Yield Fact**
When traveling to or from yellow fever-endemic areas, it is essential to ensure that all necessary vaccinations and prophylactic measures are taken to prevent the spread of the disease.

**Correct Answer: B. 1 kilometer.