The correct answer is - C. flying diagonally to longitudes.
By flying diagonally to longitudes we can make the shortest and quickest trip around the Earth. This is due to the shape of the Earth, which is rounded, so going to the longitudinal lines will be essential for having the shortest route. If we take a route that is just a circle around the the Earth, than the trip will prolong itself and the distance will increase, and this will be because in that way we will be changing longitudes, and instead of taking a precise and short route, we will end up ending at a different location from what was aimed to and plus having a longer trip.