Stage 2 (Donkey Kong Jr.)
From the Super Mario Wiki
Donkey Kong Jr. once again has to climb to the top of the area. This time chains have replaced the vines of stage 1 and, instead of the old Snapjaws, Mario throws Nitpickers at Donkey Kong Jr. Nitpickers fly around, harming Donkey Kong Jr. if he touches them. However they also bombard him with eggs, which can also damage him. Once Donkey Kong Jr. reaches the top, he has to use the key to get Mario to push Donkey Kong further away.
In the arcade release, completing this level treats the player to a cinematic where a helicopter airlifting Donkey Kong's cage is flying across the screen, Donkey Kong Jr. is in pursuit via a parasol, and the message tells the player to "Keep Going to Mario's Hideout." However, this scene was cut from the NES port due to space limitations.