Answer:
b. Acute angle.
Step-by-step explanation:
The Wright Brothers deeply analyzed the movements of birds in flight. This made them realize gradually that if the one wing of the plane meets the oncoming flow of air, it would be more acute angled and generate more weight or lift on that side leading to the fall of the plane. This was also called "the angle of attack".
Thus, they developed an alternative technique of "wing-warping" idea into their plane in order to maintain the balance through the adjustment in the angles of elevation during the flight.