Since the airplane's velocity is constant, the airplane is not accelerating in any direction. In absence of acceleration, all forces acting on the plane are balanced so that the net forces in the horizontal and the vertical directions are zero. Given the drawing, this implies that
Forward force = Drag force (F=D)
Weight = Lift (W=L)
which corresponds to option (B)
(it is not C because nothing in the text says that the vertical and horizontal components have to have same magnitude)
