Level 9

If you want to see World 9, just start again from World 1-1 and work your way to World 8-4 without warping. As soon as you beat 8-4 you will be taken to World 9-1 with only one life, and if you lose all your lives there you will have to get to it all over again. If you beat World 9-4 you will be taken back to World 9-1. Fortunately, all the levels are the easiest levels in the game.

Also, a Warp Zone is a place where you see one pipe with the text "WELCOME TO WARP ZONE!" above it. Entering the pipe will take you to another world. You have to avoid going in these pipes if you want to see World 9 (this includes reverse Warp Zones as well). If you happen to run into a Warp Zone by accident, you can purposely lose a life, and you will still be able to see World 9. In most cases, you'll have to run out of time if you accidentally see a Warp Zone, but sometimes there are pits where you can fall in (like in World 3-1).