The correct answer is C. 1 in 6.
The transitional surface is a paved area that connects the runway to the taxiway. It is designed to provide a smooth transition for aircraft as they taxi from the taxiway to the runway and vice versa. The slope of the transitional surface is important to ensure that aircraft do not experience any sudden changes in elevation as they taxi.
According to ICAO, the slope of the transitional surface at right angles to the center line of the runway should be 1 in 6. This means that for every 6 feet of horizontal distance, the surface should rise or fall by 1 foot. This slope is designed to provide a smooth transition for aircraft and to minimize the risk of accidents.
Option A, 1 in 4, is too steep and could cause aircraft to tip over. Option B, 1 in 5, is too shallow and could cause aircraft to scrape the ground. Option D, 1 in 7, is also too shallow and could cause aircraft to scrape the ground.