According to I.C.A.O. the recommended length of air ports is decided on A. Sea level elevation B. Standard sea level temperature (15°C) C. Effective gradient percentage D. All the above

The correct answer is D. All of the above.

The International Civil Aviation Organization (ICAO) is an agency of the United Nations responsible for standardizing international aviation regulations. The ICAO recommends that the length of an airport runway be determined based on the following factors:

• Sea level elevation: The higher the elevation of an airport, the thinner the air, and therefore the longer the runway needs to be to allow aircraft to take off and land safely.
• Standard sea level temperature (15°C): The temperature of the air affects its density, and therefore the amount of lift that an aircraft can generate. The ICAO recommends that runways be designed for a standard sea level temperature of 15°C.
• Effective gradient percentage: The effective gradient is the slope of the runway, taking into account the elevation of the airport and the temperature of the air. The ICAO recommends that runways have an effective gradient of no more than 1%.

In addition to these factors, the ICAO also recommends that runways be designed to accommodate the largest aircraft that are likely to use them. This is done to ensure that all aircraft can take off and land safely, regardless of the weather conditions or the weight of the aircraft.