All The Extra Lives You Can Handle
Okay, this one's a little more complex. Head to world 1-2. Go down the pipe to start the level, and then make your way to the next area where another pipe is waiting. Hit the question block to get a fire flower. Grab it, take out the Koopa Troopa. Keep firing fireballs to move the shell into the next area. Once it's all the way in the right corner, fire off a few more to get it to hop onto the purple blocks. After it's up there, fire off a few more to get the shell into the pipe.
Once the shell is transported through the tube, it'll get stuck between two walls. Head through that same tube yourself, and jump on the shell while it's bouncing around to start wracking up the 1-ups.