Rainbows appear when sunlight shines through water droplets in the air. These droplets act like small prisms and split the sunlight into different colors. But to actually see a rainbow, the sun must be low in the sky.

The perfect angle for a rainbow to form is when the sun is about 40 degrees or less above the horizon. This means rainbows usually happen early in the morning or later in the afternoon when the sun is not high overhead.

If the sun is too high—above 42 degrees—you usually can’t see a rainbow because the light wont bend the right way to reach your eyes.

Let’s review the options:
- 65° and 90° are too high.
- 60° is also too high.
- 40° is the correct angle for rainbow formation.

So, the correct answer is 40 degrees.