From the Super Mario Wiki
This article is under construction. Therefore, please excuse its informal appearance while it's being worked on. We hope to have it completed as soon as possible.
World D is the fifth and final secret world that was featured in the game Super Mario Bros.: The Lost Levels. Like the other secret worlds, it was left unused in Super Mario Bros. Deluxe. When Bowser is defeated at the end of World D-4, Princess Toadstool is saved, and, as such, Mario and Luigi beat the game.
||A level similar to World 8-1. In the original FCD version, it is a snowy level, but in the SNES remake, its a grassy level.
||This level includes several upside down Piranha Plants.
||A level which has a brick wall in the background similar to World 8-3's. In Super Mario All-Stars the wall is gone so the brick blocks are replaced with hidden blocks.
||A castle which starts out like World 8-4, but there are only 5 Fire Bars that Mario or Luigi needs to avoid.
Super Mario Bros.: The Lost Levels
Super Mario All-Stars