Super Mario World/Star Road/Star World 1
Key edit
By spin jumping along the right edge of the level you will be able to reach the key easily.
Baby Red Yoshi edit
A Baby Red Yoshi is found in the bottom portion of the level, but it's hard to get, and will need a bit too much food. If a Red Yoshi is desired, it may be easier to instead find it on really think you should use Star World 4.