World 7-6 (New Super Mario Bros.)
From the Super Mario Wiki
This level is built on multiple mushrooms, and it has many Koopa Troopas, Koopa Paratroopas, and Piranha Plant. Near the beginning of the level, the player can throw a Koopa Shell to earn a 1-Up Mushroom out of a ? Block.
At the midway point, the player has to break the middle block on the top row first. Then they have to hit the block in the same position on the bottom row for the vine to the secret exit to show. Then the player has to ride the moving mushroom-like platform across to a pipe, where the secret exit will be held. On a side note, if the player doesn't take this exit, the path on the map will lead them back to World 7-4.