The correct answer is: C. 850 knots
The air speed of an aircraft is the speed of the aircraft relative to the air. The ground speed of an aircraft is the speed of the aircraft relative to the ground.
In this case, the air is blowing at a speed of 75 knots in the direction of the aircraft. This means that the aircraft is moving through the air at a speed of 75 knots plus its own speed. The aircraft’s own speed is given as 775 knots. Therefore, the air speed of the aircraft is 775 knots + 75 knots = 850 knots.
Option A is incorrect because it is the ground speed of the aircraft. Option B is incorrect because it is the speed of the air. Option D is incorrect because it is the speed of the aircraft relative to the ground minus the speed of the air.