According to Newton's third law of motion, for every action there is an equal; and opposite reaction.
According to this law, if an object Y exerts a force on another object X, then object X exerts a force that is equal in magnitude but opposite on object Y.
Therefore, in this case, the action is the push by the ice skater against a sled at rest then the reaction is both the skater and sled moving away from each other at different accelerations.
