Stage 3 (Donkey Kong Jr.)
From the Super Mario Wiki
Mario has finally pushed Donkey Kong to the city and now, instead of vines and chains as seen in previous levels, there are poles which Donkey Kong Jr. has to climb. The enemies in this stage are Sparks instead of the other enemies in other stages. Sparks attempt to destroy Donkey Kong Jr.. Donkey Kong Jr. has to make his way up the platforms in a "Z" shape to get the key and then push the key to the cage, forcing Mario to push Donkey Kong's cage (and Donkey Kong) even further, to the very final stage of the game.