Final answer:
Air resistance decreases a jet plane's acceleration during takeoff because it is an external force that opposes the motion, resulting in a decrease in the net forward force and thus reduced acceleration.
Step-by-step explanation:
When a jet plane is speeding down the runway during takeoff, and air resistance is not negligible, it has the effect of decreasing the plane's acceleration. This is because air resistance is a force that opposes the motion of the plane and according to Newton's second law, the net force acting on an object is equal to the mass of the object multiplied by its acceleration (F=ma). If the force from the engines is constant, any additional force that opposes the motion, like air resistance, will reduce the net force and thus reduce the acceleration.
In the case of the question presented, air resistance is an external force that acts in the direction opposite to the motion of the airplane, which results in a decrease of the net forward force provided by the airplane's engines. Therefore, the correct answer to the question is: b) Decreases the acceleration.