The correct answer is: 0.2%.
The International Civil Aviation Organization (ICAO) is an agency of the United Nations responsible for standardizing international civil aviation. ICAO publishes a number of recommendations for the design and operation of airports, including the rate of change of longitudinal gradient per 30 m length of vertical curve.
For A and B type of airports, the ICAO recommendation is that the rate of change of longitudinal gradient per 30 m length of vertical curve should be limited to a maximum of 0.2%. This means that the slope of the runway should not change by more than 0.2% for every 30 m of length.
This recommendation is designed to ensure that aircraft can take off and land safely. A sudden change in slope could cause an aircraft to lose control, so it is important to limit the rate of change of gradient.
The other options are incorrect because they do not correspond to the ICAO recommendation. Option A is too low, while options C and D are too high.