This stage is Really Rolling Hills (2-4). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
At the beginning of the stage, simply walk straight ahead until you get to the part where two pipes meet. Climb the wall after the pipe as Cat Mario to claim your first green star.
Green Star 2
Once you are underground and get to the fork. Go for the lower path and enter the pipe. You will then turn on all the light floors to grab the second green star.
Green Star 3
When you are about to leave the underground area. Drop below the wheels to find a hidden area with the final stamp.
At the beginning of the stage, walk towards the right where you get to two pools of water. Hit the “M” switch to reveal the stamp.
Other Notable Secrets
While still being on the ground, there is a little gap under one of the turning wheels. Run underneath and grab the secret 1UP.