Question

A sled sliding on a flat,icy surface with a constant velocity is best described by

Answer 1

The first one is actually Newton's First Law of Motion. 

Newton's First Law of Motion is commonly stated as "an object in motion will remain in motion unless acted upon by an outside force." This sled is moving across the ice at the same constant speed and velocity meaning it will won't stop unless an outside force acts upon it. An example being someone coming and grabbing the sled or maybe the sled slips off of the ice and finds itself stuck in a pile of snow. Similarly, if the sled was not moving, it would not move unless acted upon again by an outside force. 

You believed it to be Newton's Third Law, but for future reference, his third law is more about every action having an equal opposite reaction. This wouldn't explain what is currently happening with the sled. A helpful tip into figuring out where to start in figuring out the answer I got is in the example you gave it literally states "with a constant velocity" which also ends up being in the description of Newton's First Law. Though, don't rely on that because the answer could have had a made up definition.

Unfortunately, I do not know the answer to your other question. I apologize. Hope this helped.