Let a = speed of the plane A, and b = speed of plane B. (Assume that the net effect of the wind is negligible.) From the first sentence, the distance between A and B after each travels for 1 hour is a + b miles. From the second sentence, after 45 minutes, or 0.75 hour, the distance between A and B is 200 miles. Plane a has traveled 0.75a, while plane B has traveled 0.75b. Hence a + b = 0.75a + 0.75b +200 ==> 0.25(a + b) = 200. ==> a + b = 800. Therefore planes A and B are 800 miles apart now.
When using the distance formula, you are building a right triangle whose hypotenuse connects two given points. The correct answer is C, right triangle.
