Stage 1 (Donkey Kong Jr.)
From the Super Mario Wiki
The level has Donkey Kong Jr. climbing to the top of the area using the various vines and platforms strewn around to get to the top and free Donkey Kong from the grasp of Mario. However, Mario throws Snapjaws at him, which can climb vines to get him. There is also water at the bottom of the stage, which is an instant death if Donkey Kong Jr. falls into it. Donkey Kong Jr. also dies if he falls off a ledge too far above the ground. After Donkey Kong Jr. arrives at the top of the stage, he has to get the key and push it into the keyhole, forcing Mario to push the cage further away.