Final answer:
The speed of the sled decreases if the mass doubles but the force applied remains the same, because the same force is insufficient to maintain constant speed with increased mass.
Step-by-step explanation:
When the mass of the sled doubles but the applied force remains the same, the constant applied force can no longer overcome the increased force of friction that scales with mass. This is because speed does not depend on mass directly, but rather on the net force acting on the sled. Since the force applied remains the same, the net force on the sled will also remain the same, resulting in a constant speed. Therefore, option b) The speed of the sled decreases is the correct answer, as more force will be required to keep the sled moving at constant speed. If the force remains 100N and is inadequate for the increased mass, the sled will eventually come to a stop.