Perhaps the most challenging world of this game, World D begins very unfriendly, and discriminating your small-sized hero. The Power-Up is too far to the left, and this line of bricks ends in the middle of the gap, and adding those meddling Hammer Bros., grabbing this Power-Up as a small-sized hero is nearly impossible. Defeat the Hammer Bros., and then clear the long gap by bouncing off the Green Koopa Paratroopa.
If you get a Game Over and continue from the beginning of this world, you'll have to deal with this challenging opening of the level every single time, since your hero will be small-sized.
However, if you come from another world with the Fire Suit, these first challenges should be no problem, as you can eliminate those nasty Hammer Bros. from distance, then break some of those bricks and get along the line avoiding that long gap.
It's not shown in the picture, but above and slightly to the right of the third floating brick containing a Poison Mushroom is a hidden 1-Up Mushroom block. Jump from the second floating brick, and then on the Red Koopa Paratroopa to get there.
When you bounce off of the Koopa in this section and climb the vine, you'll be on the path to a pair of bonus areas, starting with coin heaven C.
This level provides a useful opportunity to gain lives, and this will be your last chance to do it.
Because the coin heaven drops you off at the highest pipe in this section (With both a regular and upside-down Piranha Plant in the same column), you can revisit both the coin room and the coin heaven multiple times. You can collect a potential 80 coins per loop.
Make sure to defeat the Red Koopa Paratroopas here to ensure that sprite limits do not prevent the Red Spring from appearing. Otherwise, you will be stuck.
The floating block at the end of this section is a narrow but crucial platform. Carefully jump to the block and hop to the flag.
By standing next to the Bullet Bill Cannon, you can keep it from releasing shots as you wait to collect the Power-Up in the area.
A Hammer Bros. duo causes trouble towards the end of this section. Dash and jump over the Bros. and between the hammers.
One useful strategy for this level is to wait by the first group of cannons for a couple Bullet Bills to fire to the right, and then chase after them through the level. Because of sprite limits, they can prevent some of the Hammer Bros. from spawning. They also can provide useful jumping-off points to avoid the Hammer Bros.
It's not visible in the picture but there is a row of nine bricks at the end of the first wall.
In Super Mario All-Stars, however, these are replaced with nine hidden coin blocks, meaning that you have to create the safety zone against the Hammer Brothers instead of trying not to destroy it.