Pluck the Piranha Flower is the first mission of
What to do
Mario must go start the level in the huge version. He must make his way to the little island near the starting position. Then, he must climb the wall to another ledge. Once here, several Piranha Plants will appear, in which Mario must punch them in order to eliminate them. Once they are killed, the Power Star will appear.
An alternative way would be to start the level in Tiny mode, make Mario go way around past the slope with the Big Steely's and jump in the pipe, which will switch to Tiny Mode. Then, head back to the Piranha Plant area. This is a nice alternative if the player does not wish to have trouble climbing the wall.