Final answer:
To push a heavier sled past frictional limitations, Bill can push the sled slightly upwards to reduce friction by reducing the normal force. Pushing downwards or getting stronger wouldn't effectively manage the friction problem.
Step-by-step explanation:
When it comes to pushing a sled that has been limited in weight by friction considerations, the best way to move a heavier load is often by changing the angle at which the force is applied. Bill could push the sled slightly upwards to decrease the normal force and therefore reduce the friction, as friction is dependent on the normal force which is the component of weight perpendicular to the surface.
Pushing the sled slightly downwards would increase the normal force and, therefore, also increase the friction, which is counterproductive. Getting stronger would only increase the maximum force Bill can apply, but it doesn't change the frictional properties of the surface or the sled, making this option less efficient in terms of friction management.